The invention discloses an alternating current and direct current coordination control method improving the system stability, and belongs to the field of electric power system operation control. The method comprises the steps of carrying out quadratic polynomial curve fitting on the power of a connecting line after smoothing and stopping are carried out on the power of the connecting line under the condition that an alternating current and direct current series-parallel power grid goes wrong, judging whether the power increase condition or the power decrease condition is met and then increasing or decreasing the direct current power. Meanwhile, the effect is more remarkable with cooperation with an additional PSS4B excitation controller. According to the alternating current and direct current coordination control method, the regional power balance relation in the swinging process of a generator set can be effectively controlled, power angle swinging can be restrained, generator tripping can be reduced, and the transient stability level and the dynamic stability level of an alternating current and direct current system can be increased.