To provide a designing method of a welded resin molded product for reducing fracture of the welded resin molded product from its welded part to enhance the reliability of the welded part, and the welded resin molded product.

In the designing method of the welded resin molded product obtained by welding molded products composed of at least two kinds of thermoplastic resins A and B, the thickness tA of at least a part of the molded product formed of the thermoplastic resin A satisfies the following formula: tA≤{(bw×Lw)×σWAB×η}/(σA+LA) (wherein tA is the thickness of the most thin-walled part positioned vertically to the welded region of the welded molded product, bw is the width of the welded part of the welded molded product, Lw is the length of the welded part, σWAB is the welding strength of the resins A and B, η is a safety coefficient of 0.05-1, LA is the length of the tA part, σA is the strength of the resin A, σB is the strength of the resin B, EA is the longitudinal elastic modulus of the resin A, EB is the longitudinal elastic modulus of the resin B, A is the elongation of the resin A and B is the elongation of the resin B. Herein, resin strength σA≤σB, resin longitudinal elastic modulus EA≤EB, resin elongation B≤A and 5%≤A≤150% are satisfied).