Only select a thermal detector with reasonable time constant,we can obtain the information needed for different sources.Therefore,time constant is an important system design parameter,and how to measure the time constant accurately is a primary issue for system designers to concern.In order to resolve the problem of unconvenient to test and having high error rate during the time constant testing,a new test system of time constant for thermal detector is designed in this article.This system bases on semiconductor modulated laser,which replaces blackbody radiation source,and uses the technology of VC call MATLAB realizing graphic model output and time constant calculate.The results of experiments show that this method has high precision and is easy and convenient to operate.The system provides a powerful tool for scientific research and testing time constant for thermal detector.