A cesium optical pump detector for buried and abandoned chemical weapons of Japan is provided, comprising a pumping light generation module, a first optical module, a magnetic resonance module, a photoelectric detection module, and a signal processing module. The signal processing module includes an amplifying and filtering unit, a self-oscillation unit, a measuring unit, and a calculating unit connected in sequence. The amplifying and filtering unit comprises a first band-pass filter configured to coarsely measure a frequency of the electrical signal and a second band-pass filter configured to finely measure the frequency of the electrical signal. The self-oscillation unit comprises a first phase shifter connected to the first band-pass filter and a second phase shifter connected to the second band-pass filter. In the present disclosure, the intensity of a magnetic field detected can be finely measured, so that the ACWs can be found efficiently and accurately.