A hydraulic energy conversion device includes a main body being an annular member having a central hole, an output shaft mounted in the central hole of the main body with a centerline extended perpendicular to the central hole; a fixing plate located at an end of the main body and provided with a plurality of passage holes; an outer cover located at another end of the main body opposite to the fixing plate and having a central opening for the output shaft to outwardly extended through the outer cover; and a plurality of vanes circumferentially spaced on the output shaft to locate in a space defined between the main body, the fixing plate and the outer cover. The hydraulic energy conversion device is mounted in a hydraulic shock absorber to convert hydraulic energy into mechanical energy and output the same without hindering the hydraulic shock absorber from normal operation.