Schaefer form and Fox form of surplus production model are widely used in fishery stock assessment. However, the two models are two special cases of the generalized surplus production model, i.e. Pella-Tomlinson model, with two special values of the shape parameter. Because the shape parameter has a close relationship with the age structure and reproductive capacity of the population and directly affects the modeled population productivity, fixing the shape parameter at some special values would raise doubts about the results of the stock assessment models of the model. The impacts of shape parameter on stock assessment results were analyzed based on Indian Ocean yellowfin tuna (Thunnus albacares) data. The results showed:1) the shape parameter was difficult to be estimated and its best value range would be greatly different with different data sets from different time periods; 2) the shape parameter had obvious impacts on the estimate of carrying capacity and the intrinsic rate of population increase; 3) the depletion levels of the stock and the overfished or overfishing levels were enhanced with the value of the shape parameter increasing and accordingly the shape parameter had important impacts on the inference of stock status and overfished and/or overfishing levels.