In the pantograph-catenary system, the change of the surface roughness of pantograph sliding plate directly affects the current collection quality of electric locomotive. In this paper, a large number of current-carrying friction experiments have been carried out on the self-developed high-performance sliding electric contact experimental machine, and the effects of the surface roughness of the sliding friction pair on important performance parameters have been studied, including pantograph-catenary contact resistance (Rj), current carrying efficiency (η), current collection stability (δ) and the friction coefficient (µ). Further, the current conduction mechanism is revealed by the microscopic approach. In order to quantitatively reveal the effect of roughness on each evaluation index of current collecting performance, the paper formulates the functional relationship between Ra (arithmetic mean height of contour) and Rj, η, δ, µ by nonlinear fitting. On this basis, a comprehensive evaluation equation is established by the entropy weight method, and then the optimal Ra value is derived for the best pantograph-catenary current collecting performance. The experimental results show that with the increase of the surface roughness Ra of the sliding plate, Rj decreases first and then increases, while η increases first and then decreases, δ increases monotonically, and µ gradually increases. The optimal pantograph-catenary current collection condition can be achieved by reconciliation of these indices. The research results of this paper provide a theoretical basis for improving the current collection quality of pantograph-catenary systems, and is useful for material selection or new type design of pantograph sliding plates.