The invention discloses a variable valve timing mechanism for HCCI engines. The variable valve timing mechanism comprises two sets of phasers, which are respectively used for carrying out the timed continuous adjustment of an intake valve and an exhaust valve; each phaser comprises a case, an impeller and an end cover; the case comprises a circumferential wall and a sidewall positioned on one side of the circumferential wall, a cavity is formed in the circumferential wall, three bosses which are raised radially and inwardly are distributed on the inner surface of the cavity, and the impeller comprises an impeller body and three vanes; the open ends of the two sets of phasers, which face the cavities of the cases, are the references, the initial position of the intake phaser is the leftmost side, and the initial position of the exhaust phaser is the rightmost side. The variable valve timing mechanism can ensure that the intake and exhaust air distribution of an internal combustion engine can be periodically and continuously regulated within a range less than or equal to 100 plus 10 degrees CA, and on a novel HCCI/SI (Homogeneous Charge Compression Ignition/Spark Ignition) dual-modecombustion engine, the variable valve timing mechanism can meet the requirements of both HCCI combustion and SI combustion for air distribution timing.