Turbo product code (TPC) is introduced into free space optical communication (FSO) system to mitigate impact of random and long deep fade caused by atmospheric turbulence. After FSO system model is set up, the anti-burst error ability of TPC is studied. Also the algorithm of bit confidence measurement under weak turbulence and Gaussian cascaded channel is derived. The performances of TPC under weak turbulence and influential factors are analyzed. Simulations show that, TPC compensates the losses of random and deep fade effectively and makes coded FSO adapted to different turbulence conditions. To obtain more coding gain, the subcode of TPC should be insured high code rate as a priority, meanwhile its error correction ability should be considered. The number of iteration being 3～4 can achieve good balance between the performance and complexity.