1. An echo cancellation system, comprising: a speaker, for outputting a acoustic signal, which is represented as x(n); a microphone, for receiving an input audio signal, which is represented as d(n); a convergence statistic generator, receiving the input audio signal, the acoustic signal and a error signal, for generating a convergence parameter; a variable step-size generator, receiving the error signal and the acoustic signal, for generating a step size parameter in accordance the error signal and the acoustic signal; a decision device, receiving the convergence parameter and the step size parameter to output a adjusted step size parameter, wherein the adjusted step size parameter is equal to a boundary value when the convergence parameter is greater than a convergence threshold, wherein the adjusted step size parameter is equal to the step size parameter when the convergence parameter is smaller than the convergence threshold; an adaptive weight control unit, receiving the acoustic signal, the adjusted step size parameter and the error signal to generate a coefficient vector; a transversal filter, receiving the acoustic signal and the coefficient vector to generate an echo cancellation signal; and an error signal generator, receiving the input audio signal and the echo cancellation signal to generate the error signal in accordance with the difference between the input audio signal and the echoless output signal; wherein the variable step-size generator comprises: an error power computing unit, receiving the error signal, for calculating an energy of the error signal, and generating an error power signal, which is represented as {circumflex over (σ)} e 2 (n), wherein n is a time index; wherein the error signal is represented as e(n), the calculation of the error power signal {circumflex over (σ)} e 2 (n) comprises: 
 {circumflex over (σ)} e 2 ( n )=α{circumflex over (σ)} e 2 ( n− 1)+(1−α) e 2 ( n ), wherein, α is a smoothing factor; wherein the input audio signal comprises a noise signal, the variable step-size generator comprises: a noise power estimator, receiving the error signal and the acoustic signal for estimating the noise signal and calculating an energy of the noise signal to output a noise power signal, which is represented as {circumflex over (σ)} v 2 (n); wherein the step size parameter represents μ(n+1), wherein n is time index, and the variable step-size generator comprises: a first multiplier, for multiplying the error power signal {circumflex over (σ)} e 2 (n) and (1−α) to obtain (1−α){circumflex over (σ)} e 2 ; a second multiplier, for multiplying the noise power signal {circumflex over (σ)} v 2 (n) and β to obtain β{circumflex over (σ)} v 2 ; a divider, for dividing (1−α){circumflex over (σ)} e 2 (n) by β{circumflex over (σ)} v 2 (n) to obtain (1−α){circumflex over (σ)} e 2 (n)/β{circumflex over (σ)} v 2 (n); a delay element, for delaying the adjusted step size parameter μ(n+1) a unit of time to obtain μ(n); a third multiplier, for multiplying the μ(n) and α to obtain αμ(n); an adder, for adding (1−α){circumflex over (σ)} e 2 (n)/β{circumflex over (σ)} v 2 (n) and αμ(n) to obtain the step size parameter μ(n+1), which is represented as: 
 μ( n+ 1)=αμ( n )+(1−α){circumflex over (σ)} e 2 ( n )/β{circumflex over (σ)} v 2 ( n ), wherein n is time index, α is forgetting factor, β is beta parameter, wherein β>0.