Abstract: A microcomputer system based on Apple-II was developed here to realize the following functions: measuring the length of moving steel sheets which are from six rolling mills on line and displaying the results to the working fields; making classifiable statistic of products according to the measurements of length of steel sheets and printing theproduct table; modifying the setting of standard length on line as needed; inquiring about the states of production randomly.The system includes a set of periperal interfacing device and the corresponding software. Several methods (including photo-electronic coupling) for anti-outer-inter-ferences were utilized to ensure that the system can work well in the worst conditions.The system has been tested successfully in the Third Steel Plant of Shanghai.