Online dynamic crack detection of train wheels is critical to ensure transportation safety. In the existing detection system, because the electromagnetic ultrasonic surface wave technology is challenging to detect the deep cracks in the wheel that endanger the safety of the vehicle operation, the online detection of train wheel cracks needs to explore new technologies. Based on the classical piezoelectric ultrasonic technology, this article proposes a novel ultrasonic probe combined array method to meet the needs of online dynamic detection of deep cracks in train wheels. To reduce the difference in the sensitivity of the probe array, a method of drawing the distance amplitude compensation (DAC) curve is given, and the defect equivalent is judged by software gain compensation. Experiments show that the process design and method of the probe array proposed in this article can detect the deep, dangerous cracks in the radial and circumferential directions of the wheel, and the test results have acceptable repeatability and consistency. It has advantages in flaw detection coverage and data processing efficiency and is suitable for daily online crack detection of train wheels.