Local characteristicscale decomposition (LCD) has improved the empirical mode decomposition (EMD) method, but it also inherits the mode mixing of EMD. Noise assisted analysis is one of the important methods to solve the mode mixing, however, LCD is more sensitive to noise. If we adopt the white noise used in ensemble empirical mode decomposition (EEMD) as an auxiliary signal, the mode mixing cannot be effectively resolved and more false components will produce. An improved LCD method termed uniform phase local characteristicscale decomposition (UPLCD) is proposed to solve the above problem. UPLCD uses narrow wave signal with uniform phase as auxiliary signal, which can suppress the mode mixing and avoid the increase of false components caused by white noise. After the validity of UPLCD is verified by the simulation signal analysis, the proposed method is applied to the mechanical fault diagnosis by comparing it with EEMD, LCD and uniform phase empirical mode (UPEMD). The results show that the proposed UPLCD can effectively decompose the modes of rotating machinery signals and has more advantages in decomposition accuracy and interference suppression than the above three methods.