The amplitude-frequency characteristic of transmission function of APD GaAsFET analog optic receiver is usually nonideal in stopband. For this reason, the output Signal Noise Ratio(SNR) is often lowered. A method is proposed to analyze and calculate the lowered output ��SNR�� caused by the nonideal stopband characteristic. A quantitative relation is established using a low-band filter, which helps to improve the output SNR. The output noise formula of this paper is universally applicable to APD-GaAsFET analog optic receiver.