The traditional incremental conductance method has two main shortcomings: its internal parameters cannot adapt to changes in external irradiance, and it tends to make misjudgments during irradiance variations. Thus this paper proposes an improved incremental conductance method based on irradiance pre-grouping and voltage change trend discrimination. First, the optimal parameters of the algorithm in each group are pre-calculated by pre-grouping the irradiance levels, and the algorithm parameters are then assigned according to the external irradiance group, allowing the algorithm to adapt to the changes in external irradiance. Second, the cause of the algorithm’s misjudgment problem is analyzed, and potential misjudgments are predicted by examining the relationship between irradiance and power trends. The voltage trend is reversed when a misjudgment is predicted, thereby avoiding misjudgment. Finally, a photovoltaic system simulation model is built to verify the proposed method. Simulation results show that the improved incremental conductance method offers better response speed and stability.