Voltage contour visualization can help system operators grasp the overall situation of the system voltage profile and quickly identify any voltage violations. However， the overhead of the interpolation computation and graphic rendering could be an obstacle to the practical application. This paper discusses several types of spatial interpolation algorithms and their unified form. Inspired by the idea that uses different resolutions in different locations on the screen， a new grid mergence method is proposed. Adjacent rectangles with similar weight vectors are merged into larger rectangles. The number of rectangles that need processing is greatly reduced. Without sacrificing the image quality， the display speed is significantly improved.