Due to nonlinear factors existed in hydraulic system and coupled interaction from specimen, the bandwidth of position and force degrees of freedom (DOF) of hydraulic actuated parallel loading mechanism (HPLM) can be improved in the limited situation, which results in the amplitude attenuation and phase lag of response sinusoidal position/force signals. To improve the tracking accuracy of sinusoidal signals, some types of amplitude and phase control (APC), which were based on the Widrow-Hoff learning algorithm, extremum seeking techniques and so on, were proposed by some researchers. However, some shortcomings such as complex control structure, superabundant tuned parameters, and huge computation and so on, made these APC methods difficult to implement. To solve the problem, an improved amplitude and phase control (IAPC) was presented. Three characteristic parameters of sinusoidal signal, which was amplitude, phase and mean value, were estimated and regarded as control variables to be controlled separately. The methods of controller design and control parameter tuning were given. The stability of system under the proposed IAPC was analyzed. To verify the performance of the proposed IAPC experimentally, a 6-DOF hydraulically actuated redundant parallel loading mechanism under position-based hybrid position/force loading control was introduced, and its position/force close-loop frequency response characteristics were presented. The experiments containing sinusoidal position loading, sinusoidal force loading and hybrid sinusoidal position/force loading were done. The experimental results showed that the tracking accuracy of sinusoidal position/force signals could be significantly improved under the proposed IAPC with the characteristics of simple control structure and less and easily tuned parameters.