Pulse contrast is an important specification of high-intensity laser, reflecting the technical level of the laser system. As such lasers generally operate in a very low repetition rate and even in a single-shot mode, real-time single-shot measuring techniques are necessary for diagnosing pulse contrast. Compared with time-scanning measurement, the techniques for single-shot measurement are very deficient and more difficult. We present a new measuring scheme supporting high dynamic range and large temporal window simultaneously. Integrating the series of innovative technologies, we make a practical measuring prototype whose feasibility and reliability as well as the main performance have been examined by the proof-of-principle experiments. The pulse contrast can now been measured with high resolution and high fidelity at a dynamic range of about 109 within a temporal window of 50～100 ps. The invented measuring prototype has been applied successfully in the national petawatt laser system.