In order to solve the problem of the production quantity calculation instead of the estimation by the management personnel, and solve the design problem of assembly lines under seasonal demand fluctuation, some design plans and advices were presented. The newly designed assembly line was balanced and labor force was allocated. First, the production process of an oil jack assembly line was analyzed, and the standard operation time of each station was measured. Next, the oil jack assembly line was balanced and the production quantity with different takt time was calculated. Then, two new design plans were put forward under the requirements of flexible production, and the daily production quantity and average labor utilization were given as well. The research not only helps managers to solve the problem of the production quantity calculation instead of the estimation by management experience, but also solve the problem of the design of assembly lines under seasonal demand fluctuations.