The work aims to reveal the anti-shudder mechanism of tribofilm formed by friction chemical reaction between P-containing friction modifier and Ca-containing detergent. Three ATFs with different P-containing friction modifiers were tested by disc-disc tribometer, and the friction characteristics of the tribofilms formed on the steel friction disc were tested. The duration of anti-shudder property of ATF-A, B and C was 24 h, 72 h and 144 h respectively. SEM, EDX, SAM and SIMS were used to determine the chemical composition, distribution density and thickness of the tribofilms. The three kinds of P-containing friction modifiers reacted with Ca in the detergent of ATFs during friction and formed Ca-P tribofilms. The thickness of ATF-A, B and C tribofilms was 400, 900 and 1120 nm respectively. According to the SEM images, the integral value of P-element depth distribution in SAM and the surface distribution value of Pelement detected by SIMS, the density of the three tribofilms had a positive correlation with their anti-shudder durability. Although the P/Ca ratio, distribution pattern, density and thickness of the three tribofilms are different, the three kinds of Ca-P tribofilms are all helpful to keep the anti-shudder property of clutch. The P-containing friction modifier with higher reactive activity can form a thicker and denser tribofilm with high coverage on the friction surface, and the anti-shudder durability can be kept for a longer time.