To achieve the high-dynamic-range measurement for weak signal, a method of realizing contrast measurement in high dynamic range for nJ-level weak signal is proposed based on second-order autocorrelation. The effects of energy (power), phase matching, and measurement noise are theoretically analyzed. The dynamic range can be effectively enhanced via the photon-counting detection of the measurement noise, selection of non-collinear angle in phase-matching and suppression of measurement noise by focusing and filtering. Thus a high-dynamic-range measurement system is established for a weak signal. A high measurable dynamic range of 1.0×10 11 is achieved for the nJ-level weak signal when using the laser source of the Shenguang II high-energy petawatt laser, which is in agreement with the theoretical result. Furthermore, accurate screening is realized for a laser source having a contrast of 4.3×10 8. The obtained results are significant for improving the contrast of the high-energy petawatt laser system.