The invention discloses a marine riser gas intrusion monitoring device and method. The marine riser gas intrusion monitoring device comprises a ground main machine, a data acquisition and transmissionsystem, and a power supply system, the data acquisition and transmission system comprises an underwater acoustic communication device, an underwater main machine, a signal converter and an ultrasonicdevice, the underwater acoustic communication device comprises an underwater acoustic communication water borne part and an underwater acoustic communication underwater part, and the underwater acoustic communication water borne part and the underwater acoustic communication underwater part are connected wirelessly. According to the marine riser gas intrusion monitoring device, the underwater main machine and the ultrasonic device are powered by the power supply system composed of a power supply switch, a remote wake-up device and a battery pack, and thus power supply of underwater equipmentis ensured; and cable connection is not adopted between water borne equipment and the underwater equipment, but an underwater acoustic communication two-way transmission mode is adopted between the water borne equipment and the underwater equipment, a cable entering water along with a marine riser to be installed is avoided, the installation difficulty is reduced, the installation time is saved, and manpower consumption is reduced.