Aiming at the problem that the external clock synchronization method is difficult to implement, susceptible to interference and insecure under the marine condition, a time synchronization method of ship fault recording device based on interpolation algorithm was proposed. Firstly, the recording host extracted the original sampling values and the rated delay information from the packets uploaded by the merging units. Secondly, the real sampling time was restored after time correction. Finally, Lagrange once interpolation operation was used to obtain the resampling values at the synchronous time. Simulation on Matlab shows that the amplitude error can be reduced by increasing the sampling frequency appropriately, and the equivalent delay corresponding to the phase error is not more than 2 μs. Prototype test shows that the error of valid values does not exceed ±0.01% under the normal condition, of which the fundamental and 5th harmonic are less than ±0.006% and -0.5% after adding the fault signal. Moreover, both of the equivalent delays are less than -3.5 μs, and the synchronization accuracy is in accordance with the T4 level specified in IEC61850.