In the process of high power frequency-doubling, the temperature inside the nonlinear crystal will become non-uniform because of absorbing the fundamental and second harmonic waves, leading to phase mismatch, conversion efficiency reduce and output power instability. This phenomenon appears severe in the high power high repetition rate laser system. Temperature distribution inside KTP crystal was analyzed by solving thermal conductivity equation. According to temperature distribution of KTP, we have theoretically calculated the optimal phase matching angles, tolerance angles and walk-off angles of type II KTP crystal as function of temperature.