Visual real-time monitoring is the premise of low frequency oscillation control in power grids. This paper shows a visual method for the control center of power grids to monitor low frequency oscillation. It processes the PMU real-time data with incomplete S-transform, and converts the waveforms to two-dimensional time-frequency figures which shows the initial time, frequency and amplitude of each low frequency oscillation mode directly. GPUs are used to show figures and calculate FFT with the purpose of improving calculation efficiency. The results of practical cases show that the real-time characters of low frequency oscillation can be identified availably by this visualization real-time monitoring method which is helpful and suitable for practical application.