Time-to-go is an important parameter in guidance law design of air vehicles, and may determine the control gain and energy cost.This paper presents a new time-to-go estimation algorithm.Based on optimal control theory, the expression for terminal miss distance is obtained, and the correlation between terminal miss and zero-effort-miss is derived.To the intercept scene called Point of Closet Approach (PCA), a new closed-form solution to time-to-go estimation is given supposing that the evasive velocity is constant.The new estimation algorithm makes use of the included angle between relative position and relative velocity vector.Simulation result shows that:1) When the included angle is big, the PCA based method has less estimation error than traditional first-order approximation; and 2) The PCA based algorithm can also be applicable when the evasive velocity is variable.