Transmittance has an important use in the optical field.In order to a ccurately measure the transmittance of an object,this paper designs a high-pre c ision laser transmittance measurement system with large dynamic range based on t he principle of autocorrelation detection.Based on the analysis of noise,the m inimum detectable light power is calculated and the dynamic range of measurement is determined.At the same time,the improvement degree of the system′s SNR is a nalyzed and calculated.Based on the 635nm semiconductor laser as light source, the use of photoelectric diode detector,using phase-locked amplifier detecting weak signal extraction and inhibition of jamming signal,lap with a measuring sy stem,the transmittance of the sample under test is obtained by analysis and cal culation,and by changing the signal modulation frequency and time constant of p hase-locked amplifier to observe its influence on SNR,finally choose frequency modulation signal in 1000Hz,time constant of phase-locked a mplifier for 30cas es of ms measurement,experiments show that the transmissivity of the system can be measured from 0.00001% to 100%,and the measurement error range is 1%.It ca n be used for reference in the dynamic range and precision analysis of the measu rement system composed of other wavelengths or other weak signals.