1. A method for optimizing an independent micro-grid system, wherein the independent micro-grid system comprises at least diesel generators, wind generators, a photovoltaic array and an energy storage battery, and the method comprises executing on a processor of a computer system the steps of: obtaining, by a parameter acquiring device of the computer system, device parameters of the diesel generators, the wind generators, the photovoltaic array and the energy storage battery; and performing, by an optimizing device of the computer system, genetic algorithm solving and optimization by adopting a multi-objective genetic algorithm based on the device parameters of the diesel generators, the wind generators, the photovoltaic array and the energy storage battery, wherein: a quasi-steady state simulation strategy is adopted by the computer system into a calculating device executed by the processor, a preset spare capacity is reserved for the independent micro-grid system, optimization indices of the independent micro-grid system are obtained by an optimization indices acquiring device, and an optimization result is obtained by an optimization result acquiring device of the computer system by using the multi-objective genetic algorithm, optimizing, by the optimizing device of the computer system, the independent micro-grid system based on the optimization result: and wherein: the optimization indices of the independent micro-grid system comprise a present value of a total cost during a total life cycle, a loss of capacity of a load, and a pollution level; wherein the present value of the total cost during the total life cycle comprises present values of costs of all the devices in the independent micro-grid system during a whole project service life and present values of salvages of all the devices in the independent micro-grid system; the loss of capacity of a load is a ratio of capacity shortage to a whole load capacity required; and the pollution level is an annual amount of various pollutants discharged; the quasi-steady state simulation strategy comprises a hard charging strategy and a power smooth strategy, wherein in the hard charging strategy, the diesel generators and the energy storage battery alternately function as a main power supply for meeting a requirement of a net load, and the diesel generators are permitted to charge the energy storage battery; and in the power smooth strategy, the energy storage battery merely functions as a supplementary power supply and discharges when the diesel generators alone are unable to meet the requirement of the load; and the hard charging strategy comprises: calculating, by the calculating device executed by the processor, a net load of the independent micro-grid system and a net load when considering the spare capacity, wherein the net load is obtained by subtracting powers generated by the wind generators and the photovoltaic array from the load power; determining, by a determining device executed by the processor, the number of the diesel generators in operation in a present time step and calculating capacity shortage under a present load, according to the net load when considering the spare capacity, a start-up conditions of the diesel generators in a preceding time step and a maximum charging power and a maximum discharging power of the energy storage battery; and after the number of the diesel generators in operation in the present time step is determined, calculating, by the calculating device executed by the processor, actual fuel consumption of the diesel generators, a charging capacity and a discharging capacity of the energy storage battery, capacity shortage and a wasted energy in the present time step, based on the net load.