A low-heat-loss diesel engine combustor structure comprises an air cylinder piston; a slope flow guiding conical surface and a lower vortex groove are coaxially formed in the top of the air cylinder piston; a flow guiding step surface is arranged between the lower vortex groove and the slope flow guiding conical surface; the included angle alpha between the flow guiding step surface and the slopeflow guiding conical surface is 120 degrees to 165 degrees; the included angle phi between the flow guiding step surface and the top surface of the air cylinder piston is 0 to 30 degrees; upper vortexgroove is formed in the portion, outside the lower vortex groove, of the top of the air cylinder piston; the projection width H of the flow guiding step surface on the top surface of the air cylinderpiston is 1 mm to 6 mm; the slope flow guiding conical surface is connected with the inner circumference of the flow guiding step surface through a first flow guiding arc; and the outer circumferenceof the flow guiding step surface is connected with the lower vortex groove through a second flow guiding arc. Through the design, fuel and flame can expand to the middle of a combustor, fuel is combusted fully, a high temperature region at the bottom of the piston moves to the middle of the combustor, and thus heat loads and heat transfer losses of the combustor are reduced.