Availability modeling of parallel-series system is proposed with the consideration of system structure, the number of spare parts, and maintenance turnover. Therefore, the whole system composed of parallelseries system and spare parts is divided into operation system and inventory system according to its function. By introducing the average rate of arrival of spare parts to reflect the influence of the inventory system on the operation system, state transfer relationship of the operation system is given under the conditions that spare parts are sufficient and insufficient. And the binomial distribution is used as the probability distribution function of expected backorder to calculate spares shortage probability’s, further to calculate system availability with the formula of full probability. Ultimately, the example shows that, firstly, aiming at series system, the results of the proposed model is better than those of the former model; secondly, under the influence of the number of spare parts and maintenance turnover, availability of multi 1/N system is not higher than series system and the availability curve is not strictly monotonically increasing convex curve with the increase of inventory.