For the ultra-low frequency dielectric loss factor measurement method, due to the low frequency of the measured signal, the sampling time is long and the amount of data collected is large. In addition, in the case of asynchronous sampling, the FFT has spectrum leakage and fence effect, which affects the accurate measurement of the dielectric loss factor. In order to reduce the sampling time and the amount of data collected, as well as the spectrum leakage and fence effect during asynchronous sampling, an ultralow frequency dielectric loss measurement method based on Prony algorithm quasi synchronous sequence is proposed. Prony algorithm and data identification method are used to estimate the fundamental frequency of the sampled voltage signal, and Newton interpolation algorithm is used to realize the quasi synchronous interpolation reconstruction of voltage and current signals, The quasi synchronous sequence of the sampled signal is obtained, and the quasi synchronous sequence is solved by FFT and dielectric loss equivalent circuit model to obtain the ultralow frequency dielectric loss factor. The dielectric loss factor is measured under the conditions of frequency fluctuation, harmonic content change, dielectric loss angle change and noise with different signaltonoise ratio. The simulation results show that the method realizes quasi synchronous sampling in software, effectively reduces the influence of fence effect and spectrum leakage on dielectric loss factor measurement, and is suitable for accurate measurement of ultralow frequency dielectric loss factor due to short sampling time, small amount of collected data and high measurement accuracy.