With the expansion of grid-connected photovoltaic power generation, the problem of small-disturbance stability caused by the dynamic characteristics of its converter control system becomes increasingly prominent. Due to the small capacity of a single photovoltaic power generation unit, the grid-connected photovoltaic power generation system often contains a large number of photovoltaic power generation units. As a result, the model of the system is high-order. And the large numbel of photovolatic power generation units greatly increases the complexity of stability analysis. Therefore, in order to propose a relatively simple and effective stability discrimination method, a reduced-order model of small-disturbance stability analysis of grid-connected photovoltaic power generation system under DC voltage control timescale through the timescale decomposition method is established in this paper. Then the small-disturbance stability criterion of the system is deduced by applying the Routh-Hurwitz criterion on the basis of this model. From the criterion, the key factors affecting the small-disturbance stability of the system under DC voltage control timescale are analyzed. The main unfavorable affecting factors are heavy loading condition, condition of weak grid connection, improper setting of parameters of converter control systems. The correctness and effectiveness of the proposed stability criterion are verified by simulation examples. The results show that the stability criterion can effectively evaluate the small-disturbance stability of the grid-connected photovoltaic power generation system under DC voltage timescale. The evaluation method does not need to establish a complex high-order model of the system, which greatly simplifies the calculation and analysis process.