This is a study on the safety and efficacy of malic acid supplementation combined with immunotherapy for anti-tumor treatment in patients with solid tumors. The primary study objective is to determine the oral safety of malic acid; Secondary study objectives: 1. To evaluate the preliminary efficacy of malic acid in the study population. 2. To determine the recommended phase 2 dose (RP2D) of oral malic acid. Exploratory endpoints: Immune indicators including white blood cell count, neutrophil, lymphocyte, monocyte, eosinophil and basophil (count/proportion); metabolic indicators including blood glucose, triglycerides; nutritional indicators including body weight.