Triboelectric-electret nanogenerator (TENG) has recently become a research hotspot in the field of energy harvesting due to its advantages in low-cost, compact structure, broad applicability, and high efficiency. In this work, we calculated the charge transfer process and the output voltage/current/power of a contact-separation mode TENG driven by continuous periodic reciprocating movements with acceleration/deceleration processes, using symbolic computations in MATLAB. The calculated amplitudes of output voltage/current peaks of the TENG vary during multiple working cycles over time in an exponential form and gradually convergence to steady ranges. For the first time, these variations, along with the asymmetry between the output positive and negative voltage/current peaks of the TENG, were attributed to the lag of the charge transfer cycle relative to the periodic movement cycle with both theoretical and experimental evidence. A detailed investigation was conducted on the influence of the initial condition and the load resistance on the variation of TENG output voltage peaks. The optimum load resistance (Ropt) was obtained by calculating the average output power of the TENG per movement cycle 2 (AvgP). Both Ropt and AvgP can be quite different in the steady output ranges from those in the first working cycle if the TENG starts working from the contact state. These results may be important for evaluating and optimizing the output of TENGs in a steady continuous working mode, and for designing TENGs in accordance with their working environment and circuit loads.