For products with different quantity discount strategies, the seller needs to manually unify the products’ discount strategies for a multi-product quantity discount. In addition, the quantity discount in current practice often fails to give reasonable quantity discounts for multiple products. Is it possible to develop an algorithm to automatically and efficiently provide more reasonable multi-product quantity discounts while keeping each product's quantity discount strategy unchanged? This is what the authors achieved via a brand-new approach in this paper. According to the nature of quantity discount, the quantity of products should have a consistent logic with the products' prices, that is the more purchases, the higher the total discount price and the higher discount on each product that does not reach its maximum discount. And it can be assumed that the traditional discrete quantity discount should also follow this nature, it is just due to no algorithm support or the interest consideration that sellers follow the nature only on discrete discount thresholds and adopt traditional results on the other quantities. Along this line of thought, the authors define for the first time the multi-product quantity discount that can be applied to multiple products with different discount strategies, and use the rigorous mathematical method to complete the development of the algorithm which supports the continuous quantity discount and the discrete quantity discount that cover the traditional discrete quantity discount.