The invention provides a method for backward deducing a constitutive model of a welding joint based on a nanoindentation test, comprising the following steps: step 1, constructing a constitutive modelof the welding joint; 2, carry out dimensional analysis to obtain a dimensionless function between stress and strain; 3, selecting that material satisfying the constitutive relation of the step 1, and carry out finite element simulation on the process of pressing the indenter into the material; 4, determining the selected material and the corresponding stress and strain, substituting the selectedmaterial into a dimensionless function, and determining the function value corresponding to each material; 5, fitting the obtained mechanical property parameter and the function value to obtain the expression of the dimensionless function; 6, perform nano indentation test on that weld joint to obtain the mechanical property parameters of the weld joint; Step 7: The constitutive model of the welded joint is deduced backwards by using the mechanical property parameters. The invention provides a reverse inference method for welding joint constitutive model based on nano-indentation test, which has the advantages of wide applicability, low cost and high accuracy.