Abstract: To improve the control accuracy of filter rod length,taking KDF4 filter rod maker as a research object,an on-line filter rod length measurement system was designed on the basis of linear light imaging principle. The designed system consisted of three modules: length measurement module,data processing and controlling module,and man-machine interface module. The projection positions of two ends of filter rod were detected via the edge detection technology of image sensor,the collected signals were calculated by PLC and converted into the length of filter rod. The test results showed that the detection error and standard deviation of the system were lower than ±0.03 mm and 0.015,respectively;it promoted the precision of filter rod length detection and the production efficiency of filter rod maker effectively.