The analysis formula of cavity photon flux for chemical oxygen iodine laser(COIL) and NCl(a)-I all gas-phase iodine laser(AGIL) is obtained using the chemical dynamics model of continuous wave cavity. The profile of cavity photon flux along the flow direction and the total output power has a simplified analysis formula for COIL but no for NCl(a)-I AGIL due to its complexity. Comparison and estimation for COIL and NCl(a)-I AGIL have been finished as viewed from the energy extraction and volume/weight efficiency.