The range of natural radioactive α particles in water in ZnS(Ag) and the semi-absorbent thickness of β particles in plastic scintillators were calculated by theoretical formula and used as the thickness of ZnS(Ag) and plastic scintillators respectively. The energy deposition curves of β particles and γ photons with different energies in plastic scintillator were obtained by Monte Carlo simulation, and the optimal thickness of ZnS(Ag) and plastic scintillator were obtained. The preliminary design of the composite detector was completed. The results show that the thickness of ZnS(Ag) is 30 μm and the thickness of plastic scintillator is 1 mm.