Photographic developing machine has been widely used in printing business nowadays. In this paper, a developing expert system (DES) was built. The production knowledge representation, data-directed search control and context avoiding mechanism were designed in DES. DES can give appropriate developing time and temperature according to developer formulation, the type of negative and developing quality needed. On the basis of actual temperature, the developing time can be modified by DES on line in order to ensure the production quality. Experimental results show that the developing machine controlled by DES can develop negatives automatically with appropriate time and temperature.