A picosecond event timer based on high-speed ring oscillator is studied and designed. The rising edge of the signal representing the event is used to trigger the high-speed ring oscillator to generate the clock pulse signal synchronized with the event. The sinusoidal reference signal is sampled and processed by the all phase FFT algorithm to greatly improve the precision of event timing measurement. Experimental results show that in the case of a sinusoidal reference signal with 10 MHz frequency, a 14 bit ADC with 140 MHz sampling frequency, and the all phase FFT with operation point number N = 8 192, we have achieved the single-shot time interval measurement precision of 3. 16 ps rms and the time stability are better than ±0. 31 ps/ h. The results are in good agreement with the error budget based on the theoretical analysis, reaching picosecond-level event timing measurements.