The concept of traditional frequency is extended and the concept of local frequency is proposed, which makes the physical meaning of frequency clearer. The wide adaptability of local frequency is also discussed. Moreover, a novel time-frequency analysis method is presented based on local frequency. The time-frequency distribution of continuous triangular wave signal is analyzed by the novel approach. Compared with wavelet transform and Hilbert-Huang transform (HHT), the results show that the concept of local frequency is correct and the novel time-frequency approach is effective.