Due to the special application scenarios of APF, the interaction of APF, nonlinear loads and the grid may cause the harmonic oscillation issues. When the passive filter is installed near APF and operates together with APF to improve the power quality, the reactive power compensating capacitor increases the possibility of oscillation. This paper compares the harmonic stability issues of APF system when the capacitor is in the grid side and in the load side. By building the small signal model among APF, nonlinear loads and grid, the loop gain is derived. It is found that the harmonic oscillation is more likely to occur when the compensating capacitor is in the load side. To improve the stability, the existing strategy of harmonic virtual admittance is introduced and a strategy based on grid-side current feedforward is proposed. The proposed method can suppress the harmonic oscillation and reject the background harmonic perturbation effectively. Finally, simulation results validate the theoretical analysis and effectiveness of the proposed method.