With the aid of applying decomposed loads,a method is suggested to construct primary structure and obtain the primary bending stress for the tubesheet of fixed tubesheet heat exchanger. The favorable redundant rotation constraint of the cylinder to the tubesheet can be remained in the primary structure. Taking a methyl alcohol synthetic reactor as example,the mentioned method is used in the FEA to solve the problem without primary structure analysis. It is founded that the allowable load yielded by the mentioned method is comparative to that by direct limit load analysis,which indicates the feasibility of the mentioned method. It should be noted that the conode simplication of the tube and the tube hole are generally non-conservative. If the stress intensity evaluation line is located on the tube hole,a suitable stress intensity enlargement factor should be employed.