Considering the random factor of wind power, photovoltaic output and load fluctuation, sequence operation theory is adopted to handle the probablity density model of these fluctuate error with addition-type-convolution (ATC) and subtraction-type-convolution (STC). While the discrete sequence of pure load’s probability density function in each hour is acquired, the plus and minus spinning reserve of microgrid under different reliability levels that meeting the conditions such as unit climbing is optimized. Taking the minimum of the daily electricity generating operation cost and environment cost as the object function, this paper optimizes the output of controllable unit, the charge and discharge power of storage battery and interruptible load. Modified artificial bee colony (ABC) algorithm is used for figuring out the model. Comprehensive economic benefit (CEB) indicator, which considers expected energy interruption cost, expected wind power and photovoltaics cut cost, and plus and minus spinning reserve cost, is put forward. Based on the indicator, what value of the reliability indicator could be maximizing the economic benefit of spinning reserve in microgrid is synthetically evaluated. Finally, the praticability of the model and algorithm is proved by example. This work is supported by National Natural Science Foundation of China (No. 61364027) and Natural Science Foundation of Guangxi (No. 2014GXNSFAA118384).