The invention provides a piston-type direct-drive engine and a design method for first taking force and then integrating; symmetric pistons are arranged in an engine shell to directly drive a double-sided rack between the pistons to move back and forth; the double-sided rack drives two sets of gears to drive a transmission shaft to rotate back and forth to externally output two circuits of intermittent rotating force in opposite directions; and then a crankshaft connecting rod-type or a single gear-type power integrating mechanism integrates the two circuits of intermittent rotating force in opposite directions into continuous rotating force in a direction. The pistons are arranged in a cylinder tube and are connected with both ends of the double-sided rack; the upper side and the lower side of the double-sided rack are respectively provided with an upper transmission shaft and a lower transmission shaft; an upper transmission gear and a lower transmission gear are respectively arranged on the upper and the lower transmission shafts and are meshed with the upper sided rack and the lower sided rack of the double-side rack, and the upper and the lower transmission shafts are connected with the power integrating mechanism; the power integrating mechanism comprises an eccentric wheel, a connecting rod and a crankshaft; the eccentric wheel is connected with the ends of the upper and the lower transmission shafts; and both ends of the connecting rod are connected with the eccentric wheel and the crankshaft.