Prompt neutron decay constant is a key parameter in neutron action. Due to the real-time measurement demand for prompt neutron decay constant, a measurement method for prompt neutron decay constant is developed based on 252Cf Source and neutron pulse frequency spectrum measurement system. Triggered and initiated by 252Cf random neutron source, the pulse from detector is acquired and real-time converted to multi-channel data up to 1 GHz sample rate. The prompt decay constant can be acquired by utilize least squares of single exponential fitting to prompt decay curve based on prompt decay spectrum. The experimental result shows that prompt decay constant is 1.033μs-1 and the proposed method is a novel method for real-time measurement of prompt neutron decay constant.