The embodiment of the utility model provides a formation pore pressure determining device. The formation pore pressure determining device comprises a liquid level sensor, a first comparator and a receiving interface, wherein the liquid level sensor collects drilling fluid flow parameters of a slurry pump inlet, drilling fluid flow parameters of a return pressure pump inlet and drilling fluid returning flow in real time; the first comparator compares the amount relation between the downhole drilling fluid inlet flow with drilling liquid returning flow according to the drilling fluid flow parameters of the slurry pump inlet, the drilling fluid flow parameters of the return pressure pump inlet and the drilling fluid returning flow which are collected in real time, and outputs the amount relation to a computing device outside the formation pore pressure determining device; the receiving interface receives the formation pore pressure transmitted by the computing device in real time, the formation pore pressure is obtained in the mode that the computing device computes the difference between the formation pore pressure and downhole drilling fluid column pressure according to the amount relation between the drilling fluid inlet flow and the returning flow and the formation condition information and then computation is carried out according to the known downhole drilling fluid column pressure. According to the scheme, accuracy of a formation pore pressure computation result can be improved.