This paper investigates a specific current saturation phenomenon in high-voltage LDMOS (lateral double-diffused MOS) transistors, i.e., the quasi-saturation effect. By means of simulation tool TCAD, we clarified that the physical mechanism of quasi-saturation effect is the carrier velocity saturation in the drift region. Further more, with the concept of intrinsic drain voltage Vk, a mathematical model is derived to describe the current saturation effect in LDMOS transistors. The proposed model has been primarily validated by Matlab program and exhibites excellent accuracy, speed and scalability. This model can be further implemented in SPICE circuit simulation.