Vermicular graphite percent is an important index of evaluating the vermiculation of graphite in cast iron. Quantitative metallographic method is commonly used to test the vermicular graphite percent, but it can only detect certain parts of the cast iron specimen. Therefore, an ultrasonic system was designed to detect the degree of the vermiculation in cast iron according to the relationship between the ultrasonic propagation velocity and the vermicular graphite percent, and it is composed of the sink filled with ultrasonic coupling agent, ultrasonic emitting probe, ultrasonic receiving probe, high-frequency signal generator, ultrasonic signal acquisition unit and computer. Without measuring the length of the cast iron specimen, the propagation velocity can be calculated by three different kinds of propagation time. A specific relation between the vermicular graphite percent η and the propagation velocity ν was got by experiments. When the propagation velocity is below 5700 m･s⁻¹, they have an approximate exponential relationship, namely η = 0.6·exp(26800/ν). When the propagation velocity exceeds 5700 m･s⁻¹, they have a linear relationship, namely η = −ν/1.53 + 5800/1.53. The comparative experiments showed that the coefficient of variation and the error were below 0.3% and 4%, respectively, which indicated the repeatability and accuracy of the system are very good.