Abstract: A computation algorithm for simultaneous chemical and phase equilibrium is proposed based on minimization of Gibbs free energy. By using the extent of reaction and the yield factor of phase, the computation of simultaneous chemical and phase equilibrium is transformed as into unconstrained nonlinear minimization problem, and is solved by Genetic Algorithm. The proposed algorithm has a wide application prospect because in this algorithm the variables are less,and negative mole-fraction, which usually appears in conventional algorithms,will not occur,and it can obtain a global optimization solution. The formaldehyde-water system is taken as an example.It is found that the computation results are just the same as those reported in the literature, which shows that the presented algorithm is robust and efficient.