Against the deficiencies of traditional time domain and frequency domain analysis in detecting wheel-rail(W-R) system hidden risks which wheel flats generate,the time-frequency characteristics of W-R shock caused by wheel flat are analyzed and the vehicle-rail dynamic model with wheel flat is investigated.The 10 degrees of freedom(DOF) vehicle model is built up.90-DOF rail model is constructed.The wheel flat excitation model is built up.The vehicle-track coupling dynamic model including wheel flat excitation is set up through nonlinear Hertzian contact theory.The vertical accelerations of axle box are calculated at different speeds and flat sizes based on the vehicle-track coupling dynamic model with wheel flat.Frequency slice wavelet transform(FSWT) is employed to analyze timefrequency characteristics of axle box accelerations to detect the W-R noncontact risks,which the traditional time domain or frequency domain method does not analyze.The results show that the small flat size and high running speed lead to high frequency W-R impact.Large flat size and high running speed result in momentary loss of W-R contact,and there exist security risks between wheel and rail.The conclusion that the phase of axle box accelerations is same to W-R forces lays a theoretical foundation of monitoring W-R contact safety from axle box acceleration instead of traditional W-R force detection. Against the deficiencies of traditional time domain and frequency domain analysis in detecting wheel-rail (WR) system hidden risks which wheel flats generate, the time-frequency characteristics of WR shock caused by wheel flat are analyzed and the vehicle-rail dynamic model with wheel flat is investigated. 10 degrees of freedom (DOF) vehicle model is built up. 90-DOF rail model is constructed. The wheel flat excitation model is built up. The vehicle-track coupling dynamic model including wheel flat excitation is set up through nonlinear Hertzian contact theory. The vertical accelerations of axle boxes are calculated at different speeds and flat sizes based on the vehicle-track coupling dynamic model with wheel flat. Frequency slice wavelet transform (FSWT) is employed to analyze time frequency characteristics of axle box accelerations to detect the WR noncontact risks, which the traditional time domain or frequency domain method does not analyze.The results show that the small flat size and h high running speed lead to high frequency WR impact. Large flat size and high running speed result in momentary loss of WR contact, and there exist security risks between wheel and rail. conclusion of the phase of axle box accelerations is same to WR forces lays a theoretical foundation of monitoring WR contact safety from axle box acceleration instead of traditional WR force detection.