An adjustment circuit for measuring an optical parameter is disclosed. The adjustment circuit includes an optical sensing module for detecting an optical signal and converting the detected optical signal into a voltage signal; an amplification module for amplifying the voltage signal; an A/D conversion module for converting the amplified voltage signal into a digital signal; a control module for analyzing the digital signal for generating an analyzed result; a signal generating module for outputting a frequency square wave signal according to the analyzed result; and an adjustment module for adjusting an amplification factor of the amplification module according to the frequency square wave signal. The circuit of the present disclosure is easy, and an automatic measurement can be implemented.