The trypsin-assisted extraction of polysaccharides from L. was optimized using the response surface methodology (RSM). The optimum extraction conditions were extraction temperature, extraction time, extraction pH, and enzyme amount of 37.16°C, 180 min, 8.57, and 5.16%, respectively. Under the optimized conditions, the yield of L. polysaccharides (ACP) reached 9.69%, which was comparable with the predicted yield (9.73%). Mid- and high-dose ACP significantly inhibited the tumor growth (43.93%) and the tumor inhibition percentage (38.05%), which were more than 30%. The ACP could extend the survival time of H ascites tumor-bearing mice. Furthermore, the ACP could reduce the thymus and the spleen atrophy and significantly promoted the Con A-induced proliferation of splenocytes and elevated the serum IFN-γ and IL-2 levels. Therefore, the ACP could inhibit the tumor growth in tumor-bearing mice and regulated the immune function of mice. The trypsin-assisted extraction has high efficiency, is carried out through the polysaccharide extraction and the deproteinization at the same time, and is more convenient and fast than traditional methods. No detailed study on the optimization of the trypsin extraction of onion polysaccharides is available. Thus, this experiment aims to use the BBD (4 factors and 3 levels) to optimize the roles of extraction temperature, extraction time, extraction pH, and amount of enzyme on the yield of polysaccharides obtained from the fruit of L. In addition, when looking for high-quality biological functional principles for the pharmaceutical industry, the antitumor activity of ACP was evaluated. L. is one of the most widely cultivated and consumed crops worldwide. Polysaccharides are the main active ingredient, and studies have shown that a high intake of vegetables is associated with reduced risk of cancers.