The invention discloses a compensable water supply pipe biological film online monitoring device based on photoelectric effect. The device comprises light-emitting systems, optical fibers, a monitoring system, a photoelectric effect system, a compensation circuit system, circuits and a display, wherein the two light-emitting systems are connected with the monitoring system, the monitoring system is connected with the photoelectric effect system, the photoelectric effect system is connected with the compensation circuit system, and the compensation circuit system is further connected with the computer display; the light-emitting systems comprise light-emitting pipes and light couplers; an emitting-receiving combined probe, an emitting probe and a receiving probe of the monitoring system are all fixed on the pipe wall through fixing devices; the photoelectric effect system comprises two groups of orderly connected light couplers, CCD image sensors and amplifiers; the compensation circuit system comprises two A/D converters respectively connected with a subtracter. The device is suitable for the continuous online monitoring of growth status of the biological film on an urban water supply pipe, the response speed is fast, the interference factors are few, the influence caused by the environment factor is compensable, the stability is good, and the accuracy is high.