A novel method to calculate the thermal focal length of corner-pumped slab lasers is proposed. Traditional methods generally use uniform distribution or exponential decay distribution to characterize the heat distribution, and the difference between calculation results and the experimental value is not small which is against the further cavity design. Tracepro software is used to trace the pump light in the medium so as to get the accurate heat distribution within the gain medium, and then the temperature distribution and thermal focal length is calculated. The calculation results of this method are according with experiments much better than those of the traditional. This method is of general applicability, and is especially suitable for lasers with novel pump ways and special shape gain media.