In many energy spectrometer detectors, baseline of nuclear pulse is usually not zero. This baseline drift is mainly caused by power supply instability, component error or other factors. Nuclear pulse waveform is often exponential. Because exponential waveform curve is not a straight line, its baseline is difficult to measure accurately by means of direct measurement. Especially when the pulses pile up seriously and noise is severely, the baseline measurement will be more difficult, and the energy spectrum of the radiation cannot be measured accurately either. In this paper, a way of nuclear pulse baseline subtraction based on trapezoidal shaping is put forward. The exponential nuclear pulses are shaped to trapezoidal ones, so the tail of the pulse curve will be converted to a straight line. Straight top line and baseline can make the net amplitude of the pulse be measured accurately, and the energy resolution of the spectrometer can be improved.