A constant value method for detecting and evaluating internal leakage in a hydraulic cylinder and a detection device. The method comprises: step (1) measuring an accurate pressure drop value Δp of the internal leakage in the hydraulic cylinder; step (2), determining whether the detected hydraulic cylinder meets a standard: if the product of the accurate pressure drop value Δp of the internal leakage in the hydraulic cylinder and a corresponding hydraulic stroke L, is less than a standard parameter C for evaluating the internal leakage in the hydraulic cylinder,





C
=


4
⁢
QtK


π
⁢

D
2




,




the internal leakage in me hydraulic cylinder meets the standard. Otherwise, the internal leakage in the hydraulic cylinder does not meet the standard. The detection device comprises a motor, a hydraulic pump, a directional valve, a detected part, a pipeline disassembly joint and a pressure sensor.