Abstract: In view of problem that mechanical and non-digital temperature and humidity instruments cannot upload data, a digital temperature and humidity instrument taking single-chip microcomputer AT89C51 as control core was designed. Hardware and software design of the instrument were introduced in details and programming implementation of three operation functions of SHT11 were given, namely reset sequency function, writing byte sequency function and reading byte sequence function. The test result showed that the instrument has high reliability and low cost.