The influence of thulium ion concentration and output-mirror transmittance on Tm:YAP crystal laser output performance is experimentally investigated. Three c-cut crystals in size of 3 mm×3 mm×5 mm with 3%, 4% and 5% Tm3+ (ionic-number fraction) are examined with the transmittance of 5%. Experimental results showed that the best power performance is obtained with the 4% Tm:YAP crystal. By using 24.8 W incident pump power, a maximum output power of 7.5 W is obtained. The slope efficiency is 48.8%. Using different output transmittances of 2%, 5% and 10%, we investigate the laser performance with 4% thulium ion. The results show that with the transmittance of 5%, the highest slope efficiency is obtained.