Abstract: Ameson portunus is one of the common pathogens of the swimming crab (Portunus trituberculatus). In this study, a SYBR Green I real time qPCR method whose amplification efficiency reached up to 101.45% with a detection limit as low as 2.0×10¹ copies·μL^-1 was established by targeting A.portunus SSU rRNA gene. The method was A.portunus-specific with good repeatability within and between groups. Then the qPCR method was used to detect farmed swimming crabs from a "toothpaste disease" epidemic area, and the detection rate of A.portunus was 82.35%. The results show that the average muscle load per milligram of the live crabs was lower than that of the dead crabs significantly, which indicates that the death of crabs in the region is related with infection of A.portunus closely. When the load of A.portunus was 1.0×10⁹ copies·mg^-1 in the muscle tissue, P.trituberculatus had a risk of illness or even death. Besides, the detection rate for the same samples by a previously established nested-PCR method was only 64.71%, showing that the qPCR method in the present study had higher sensitivity and can provide good technical support for the early diagnosis and prevention of "toothpaste disease".