The principle and design of a new combustion system (DSCS, Double Swirls Combustion System) are provided in this paper. New theory for fuel-air mixing and burning in DSCS is found by means of fuel jet modeling experiments. The flow process of fuel jet can be divided into four phases in detail. Fuel jet collides with the ridge of DS combustion chamber, then splits and forms double swirls, etc. So, mixing and burning can be carried out more efficiently. The test results of the single-cylinder test engines and the commercial engines showed that DSCS had many advantages.