An ultrathin wearing course (UTWC) is a preventive maintenance treatment method for a road surface which can improve the safety and comfort of driving by providing superior skid resistance. However, the skid resistance is a concern after multiple years of loading cycles. The role of coarse aggregate in preventing the skid resistance deterioration of an UTWC is not fully understood. This study determined the effect of coarse aggregate types and nominal maximum aggregate size (NMAS) on the skid resistance deterioration of an UTWC. Different types of stone mastic asphalt (SMA), which are used in three kinds of coarse aggregates (diabase, limestone, and basalt) and three kinds of NMAS coarse aggregate (SMA-5, SMA-8, and SMA-10), were prepared for the test. A Model Mobile Load Simulator 3 (MMLS3) was used to simulate repeated vehicle loading and abrasion. After specific loading cycles, the skid resistance of an UTWC was measured by the British pendulum number (BPN) test and the sand patch test. The BPN and mean texture depth (MTD) were chosen to evaluate the skid resistance of the UTWC. The results showed that the BPN value of an UTWC is sensitive to the type of coarse aggregate. The skid resistance of SMA-8 with diabase and basalt is better than the performance of SMA-8 with limestone. The results of grey relational analysis (GRA) showed that the loss polishing stone value (LPSV) index should be used to evaluate the coarse aggregate of the UTWC. The MTD value of an UTWC is sensitive to the NMAS. The skid resistance of SMA-8 and SMA-10 is better than that of SMA-5. The skid resistance deterioration (BPN and MTD) of UTWC was fitted by an exponential model. The measured data of the Guangxi Nanyou expressway for 5 years showed that the exponential fitting equation can describe the attenuation process of the skid resistance of an UTWC in actual engineering. The findings will play an essential role in selecting coarse aggregates in an UTWC.