Aiming at the optimal size of distributed generation in isolated hybrid microgrid that consists of wind/ photovoltaic/diesel/battery, using the reliability of power supply as the constraint condition, this paper constructs a microgrid multi-objective optimal model which takes economy, environmental protection and energy efficiency into consideration. The entropy theory is introduced to determine the weight coefficient of multi-objective function to get rid of the subjectivity of weight value effectively. Moreover, self-adaptive steps, adaptive variation, reduction of search space and other measures are adopted to improve the convergence of the electromagnetism-like mechanism (ELM). Based on improved cycle charge (ICC) control strategy, the improved ELM is used to solve the optimal model. Taking Guangxi Weizhou Island as the background for a case study, the effectiveness of the model and algorithm is verified. The results of simulation show that such optimization problems with a lot of constraints and huge computation, such as microgrid planning, can be solved by this algorithm. In addition, the proposed algorithm has obvious advantages in global optimal capability and the speed of optimization.