In order to further reduce the power fluctuation of grid-connected photovoltaic arrays, this paper proposes a power smoothing method that used in photovoltaic array connecting to the grid based on variable-boundary tracking differentiator. Besides simulated annealing particle swarm optimization is used to optimize the boundary parameters. Firstly, a generalized form of variable-boundary tracking differentiator is constructed according to variable-boundary theory. Then, for the boundary parameter optimization problem, this paper proposes a fitness function based on short-time-scale volatility for real-time optimization, which improves the smooth ability of the algorithm. Taking a photovoltaic array in East China as an example, the experiment proves that this strategy has a better smoothing effect than the traditional strategy. The maximum fluctuation and average fluctuation of power are reduced by 64% and 16% respectively. And the charge and discharge energy of energy storage is reduced by 5%.