A method of controlling combustion of a gas appliance includes the following steps: a) Read a first burning data in a database; b) Burn gas according to the first burning data; c) Obtain a burning efficiency of the gas appliance; and d) Compare the burning efficiency with a predetermined value, and repeat the step b to the step d when the burning efficiency is higher than the value, or read a second burning data in the database and burn gas according to the second burning data when the burning efficiency is lower than the value. The present invention provides plural stages of burning according to the main component of the gas to be burned to increase the total burning efficiency.