The invention relates to a timing system of a double-chain engine. The timing system comprises chains, double rows of inlet camshaft chain wheels, double rows of exhaust camshaft chain wheels, double rows of crankshaft chain wheels, a fixed guide rail, a movable guide rail and a tensioner, wherein two rows of inner and outer sprockets are respectively arranged on the double rows of inlet camshaft chain wheels and the double rows of exhaust camshaft chain wheels, the two chains are longitudinally staggered respectively and sleeve sprockets of two rows of inner and outer chain wheels of the double rows of inlet camshaft chain wheels, the double rows of exhaust camshaft chain wheels and the double rows of crankshaft chain wheels, the pitch of the chains is P, the longitudinal staggered distance of the two chains is d which is larger than 0 and is less than or equal to P/2, the fixed guide rail and the movable guide rail are respectively positioned on both sides of the chains, and the tensioner is connected with the movable guide rail. The timing system has the beneficial effects of low noise, low cost and long service life.