Beam spreading is the foundation of proton therapy for large area and depth tumor. The characteristics of proton lateral beam distribution for a solid beam stopper double scattering system are investigated using Monte Carlo multi-particle transport code Fluka and the results are compared with analytical computation by using Highland formula under the same conditions. It is found that proton fluence obtained by Fluka is lower than that by analytical computation with Highland formula in the region where the variation in uniformity is ±25% and they coincide better with thin-foil than with thick-foil. For a solid beam stopper double scattering system，as the thickness of foils increases，the radius of beam spreading increases，but the efficiency of beam use age increases firstly and then decreases，the maximum value is about 169%.