The invention belongs to the technical field of seabed monitoring and discloses a seabed in-situ excess pore pressure long-term monitoring device based on a casing pipe drilling mode. The seabed in-situ excess pore pressure long-term monitoring device comprises a detection device and a data processing device; a seabed drilled hole is formed in the longitudinal direction of a seabed; the detectiondevice is sequentially provided with a probe, a hydraulic pipe and a sensor from bottom to top, the probe and the hydraulic pipe are placed in the seabed drilled hole, the seabed drilled hole is filled with sealing slurry, the sensor is arranged on the upper surface of the seabed, the top end of the hydraulic pipe is connected with the sensor, and the sensor is connected with the data processing device through coaxial cables; and the probe is provided with a water-permeable part and a hydraulic pipe connecting part from bottom to top, wherein the water-permeable part and the hydraulic pipe connecting part are connected mutually, the lower end of the water-permeable part penetrates into a deeper seabed at the bottom of the seabed drilled hole, and the upper end of the hydraulic pipe connecting part is connected with the hydraulic pipe; and the water-permeable part, the hydraulic pipe connecting part and the hydraulic pipe are all hollow to form hydraulic transmission channels. The seabed in-situ excess pore pressure long-term monitoring device can measure the pore water pressure at any height of the seabed, long-time monitoring can be realized, and the defect that monitoring is conducted by a probe rod in the prior art is overcome.