Abstract: It is assumed that the escaping process of atmosphere on planet is a quasi-static process. The escape equation of atmosphere is derived based on such assumption. Half life time due to escaping rate of atmosphere on planet are defined. The expressions are given. The half life times of atmosphere on Earth and Venus for different temperature distributions are calculated from the formula. It is shown the half life times of atmosphere for Earth and Venus are much greater than the universe age（～10¹⁰y）,the atmospheres for both are very stable. The half life time of atmosphere for other planet and moons can also be calculated from the theory.