The harmonic conversion rule of I/II angle-tuned third harmonic gemeration (THG) system will be far away from the ideal rule on actual large-aperture and high-intensity laser facility. To study actual harmonic conversion rule and guide the crystal debugging, the nonideal conditions, such as 1ns Gauss pulse, different wave radius and crystal loss have been studied. It is proved that the THG external efficiency will reduce 10% than flat-temporal pulse. To obtain the highest THG efficiency, the optimal SHG internal tuned angle is not 220 μrad but 160 μrad impacted by 1 ns Gauss pulse and crystal loss. And the optimal second harmonic generation (SHG) internal efficiency is not 67% but 60%~63%. The higher of crystal loss, the lower of SHG internal efficiency. And the higher of 1ω intensity, the greater of infection of wave radius and divergence angle on harmonic conversion.