A numerical model of electro-absorption-modulator (EAM)-based time-of-flight depth system is established. A formula with scale factor is deduced to quantify the contribution of timing-error to the accuracy of system. Measurement error is studied with parameters of EAM as well as background noise and timing-error. The results show that with no timing-error the standard deviation of measurements is inversely proportional to the square root of the number of signal electrons and is proportional to the square root of the ratio of the number of background electrons to the number of signal electrons. System accuracy improved by employing EAM with high speed modulation and higher extinction ratio. With skew error increasing, measurement error rises rapidly and is difficult to be reduced by increasing the number of signal electrons in the well of sensor. The well capacity of sensor must be larger than 300 Ke and the skew error of signal must be less than ±200 ps if the accuracy of 7 m single depth image is required less than 1 cm.