1. A remote monitoring system for remotely monitoring execution status of a Programmable Logic controller (PLC) program of a machine, comprising: a storage device, storing ladder diagram information corresponding to a PLC source program of the machine, wherein the PLC source program is generated by compiling a ladder diagram and the ladder diagram information includes a PLC signal address relation information, configurations of a plurality of logic switches for the ladder diagram and a responsive collect command of each logic switch of the ladder diagram for retrieving parameter data corresponding thereto, and the PLC signal address relation information indicates the relations of the logic switches on the ladder diagram; a parameter retrieval unit, respectively retrieving parameter data corresponding to the logic switches from the machine using the responsive collect commands of the logic switches; and a monitoring unit, generating a status of ladder diagram according to the configurations of the logic switches, the parameter data and the PLC signal address relation information, wherein the status of ladder diagram represents the parameter data corresponding to each logic switch of the ladder diagram when the machine is executing the PLC source program so as to remotely monitor execution status of the PLC program of the machine.