The invention discloses a method for evaluating a least clearance between an engine piston and a cylinder head. The method comprises the following steps of preparing an engine in a normal state, and performing machining on top of the piston for arranging a threaded hole, mounting a screw plug on the threaded hole position at top of the piston; manually rotating the engine, measuring a least static clearance h1 between the piston and the cylinder head; mounting the engine on a testing station frame, and measuring a least thermal dynamic clearance h2 between the piston and the cylinder head; calculating a thermal expansion value and a force bearing extension value by means of a difference between the least static clearance h1 and the least dynamic clearance h2 according to the temperature, and furthermore obtaining an elevation value h4 of top surface of the piston; and on the condition that a least value h3 of the clearance between the piston and the cylinder head through calculation according to a dimension chain is larger than the elevation value h4, using the least thermal dynamic clearance h2 >=(0.2%*piston stroke) as a design standard for evaluating the least clearance between the piston and the cylinder head. The method can realize accurate evaluation for the least designed clearance and furthermore can be used for evaluating a piston-and-top collision risk on an engine market.