Explosive loading was one of the loadings of warship structural under anti-ship missile.The computing method of explosive loading inside closed cabin was the key of accurate calculating the response of stiffened plates.Warship cabins were made up by stiffened plates and stiffeners on one side of siffened plate,so buikheads were palte or stiffened plate.Numerical method was used in studying the influence of flat steel on loading of ecplosion inside closed cabin.Flat steel has obvious influence on explosion loading inside cabin,converging shock waves were formed on the side towards blasting source.The diffraction effect of explosion shock wave on another side of flat steel was analyzed,and reflceted shock waves of bulkheads were propagating between buikheads until the pressure of cabin become equivalent.