For high power thin disk laser (TDL), the anti-amplified spontaneous emission (ASE) cap is one of the effective methods to suppress the ASE effect. Thermal aberration and dioptric power of the capped disk was stronger than that of the uncapped disk. With a particular anti-ASE cap thickness the dioptric power of the capped disk almost keeps constant with the increase of pump power density. However, the pump spot uniformity of a thin disk laser will be changed if the anti-ASE cap is thick enough. A numerical model developed to analyze the pump spot uniformity for different thickness anti-ASE caps. The analysis results show that the thickness of the anti-ASE cap will deteriorate the pump spot uniformity. Thus, weighing pump spot uniformity will be important for designing an anti-ASE cap (YAG, n=1.82) with an optimal thickness to realize higher-power laser generation.