Area density is a an important parameter for measuring compression in inertial confinement fusion experiment. Using the MULTI code, physical parameters of fuel plasma are simulated at time of maximum suppression in indirectly driven implosion experiment at Shenguang Ⅱ. Secondary proton energy range in CR-39 is calculated. The average area density 〈ρR〉 of fusion fuel is obtained by measuring secondary protons from D2-filled fuel in the back of CR-39 for the first time. The result shows that CR-39 can be used to measure secondary proton yields of Shenguang Ⅱ. Secondary fusion reaction is affected largely by the suppression state in indirect drive.