In traditional Chinese medicine, tongue moisture plays an important role in tongue diagnosis.  However, there are still some problems in conventional measurement. Therefore, a hygroscope, which uses digital humidity and temperature sensor, was developed to test tongue moisture based on mass transfer means.  This paper presents its testing principle and the design method of its hardware and software. This hygroscope finds itself with some attractive features, such as portability, handy operation, low power consumption, etc. It has been proved that the standard deviation of this tester, only 2.9%, is better than that of other conventional instruments, and the tongue moisture could be measured rapidly and accurately.