Abstract: The ship radiated noise detection algorithm in passive sonar system is widely studied, and the line spectrum is a feature which mostly used for ship radiated noise detection. Parameters, such as number and frequency of line spectrum, are usually needed for ship radiated noise detector which based on line spectrum character. In reality, these parameters are difficult to acquire from unknown ships. The demand of novel universal detector whose detecting performance could not be easily influenced by line spectrum parameters is urgent. The peak-like shape feature of continuous spectrum of ship radiated noise is used as the basis of the detector proposed in this paper. Application of instantaneous frequency analyzing method is extended from narrow band signal to wide band signal. An approximate formula which could demonstrate the relationship of the instantaneous frequency and the frequency and energy of the wide band signal is deduced. Using the approximate formula, the performance of the instantaneous frequency based detector for ship radiated noise is analyzed theoretically. Both simulated signals and practical signals are used to test the performance of the instantaneous frequency based detector, the results verified the conclusions gained from theoretical analysis.