The invention discloses a piston which comprises a piston head, a piston skirt and a piston pin boss. A connecting rod hole where a connecting rod penetrates through is arranged on a pin hole wall of the piston pin boss. The part of the pin hole located on the same side of the connecting rod comprises a horizontal section far from the connecting rod hole and an expanding section close to the connecting rod hole. The intersected line of the inner wall of the expanding section and the plane passing through the axis is an involute, and the aperture of the expanding section in the direction far from the horizontal section increases gradually. Thus, when the piston is in high speed movement, the piston pin is bended and deformed, and the position of the piston pin connected with the connecting rod is bended and fit with the expanding section, and the stress concentration at the intersection of the pin hole and the connecting rod hole can be reduced, so that deformation at the intersection can be alleviated and the rejection time of the piston pin boss can be postponed, as a result, the service life of the piston is prolonged correspondingly. The invention further discloses a marine diesel engine provided with the piston.