The present invention discloses a valve convenient for leakage check and a combustion gas system of a gas engine comprising the valve. The valve comprises: a housing having an inlet portion and an outlet portion; a through hole by means of which the inlet portion communicates with the outlet portion; a valve closure member by means of which the through hole may be adjusted; at least two scavenging gas troughs that are annularly disposed on end faces of the inlet portion and the outlet portion, respectively, and that are used to communicate with a scavenging gas cavity; first seal rings disposed at inner sides of the scavenging gas troughs and used to achieve seal connection between the through hole and a combustion gas passage; second seal rings disposed at outer sides of the scavenging gas troughs and used to achieve seal connection between the scavenging gas troughs and the scavenging gas cavity; and a scavenging gas passage disposed within a body of the housing and communicating with the scavenging gas troughs. The valve is disposed on a pre-combustion gas pipe; when the valve is opened, the pre-combustion gas pipe is unblocked, and the gas engine may operate normally; and when the valve is closed, the pipe is closed, and a leakage test may be carried out.