Tunnel construction drainage and sewage diversion is an important engineering technology to reduce tunnel construction sewage discharge. However, the leaching substances of unstable tunnel shotcrete can affect the drainage water quality. Therefore, the study of the influence of leaching substances on water quality during the stabilization process of shotcrete is of great significance for the precise implementation of sewage separation technology. The concrete test blocks are prepared using shotcrete materials from tunnel construction. Firstly, the leaching components were analyzed through static experiments. Then, the effects of curing times, the ratio of water to concrete (L/S ratio), and the contact areas on the pH and alkalinity of the leaching solution were explored through 14 days dynamic leaching experiments continuously. The leaching process and mechanism were also explored combined with characterization analysis. The static experimental results indicate that the large release of alkaline substances in shotcrete leads to a rapid increase in the pH value of water, which is the main reason affecting water quality. The dynamic experimental results show that when the curing time of the concrete is less than three days, the calcium hydroxide crystal inside the concrete is not solidified by the C-S-H gel, which is easy to continue to precipitate, leading to the pH value of the contact water sample exceeding the standard. With the increase of curing time, the leaching alkaline substances decrease significantly. When the maintenance time reaches 28 days, the pH value of the leaching solution drops below 9, which can meet the requirements of comprehensive sewage discharge. When the L/S ratio is low, the leaching substance has a significant impact on the pH and alkalinity of the water sample. When the L/S ratio exceeds 40:1, the leaching substance has no significant impact on the water quality of the water sample. The larger the contact area between concrete and water samples, the easier it is for alkaline substances in concrete to dissolve and precipitate, and the more significant the impact on the pH and alkalinity of water quality. This study provides important theoretical basis for the on-site construction design of drainage and sewage diversion technology in tunnel construction.