The invention relates to a spectrum detection system for diesel SCR ammonia pollution. The spectrum detection system comprises a photoelectric device unit and a gas path unit, wherein the photoelectric device unit comprises a laser, a splitter, a collimator, a laser driver, a signal generator, a photoelectric detector, a preamplifier, a lock-in amplifier, a data acquisition card and a host computer; the gas path unit comprises a gas pool, a standard gas module, an SCR purification system and a filtration system. The spectrum detection system realizes functions of real-time concentration monitoring, concentration data storage and display and the like of ammonia by using the TDLAS technology as the theoretical basis and combining with the reflecting pool and harmonic detection technology; the spectrum detection system is simple to operate, provides a solid technical basis for the field of detection and application of diesel ammonia pollution, realizes a detection and evaluation system for diesel ammonia emission, greatly improves the system detection accuracy and response speed, and provides new means for the research of emission reduction of a diesel and environmental monitoring.