This article aims to explore the significant impact of investor psychology on investment decisions, taking overconfidence and risk tolerance theories as examples to analyze their influence on decision-making. The examination of the investment psychology during the Chinas 2018 A-share crash highlights that investors are susceptible to psychological biases and emotional reactions in the face of market changes. These biases include representativeness bias, where investors may rely too heavily on stereotypes, leading to misjudgments; panic, which can trigger impulsive and irrational selling; overconfidence, causing investors to overestimate their abilities and knowledge; and catastrophic thinking, where investors focus excessively on potential worst-case outcomes. These psychological factors contribute to increased market volatility and can lead to irrational judgments and behaviors among investors. Such behavior is detrimental to the market's stable development, as it can result in mispricing of assets and increased risk of misinvestment. The article emphasizes the critical role of investor psychology in rational decision-making and market efficiency. By recognizing the personal and environmental drivers of these biases, investors can adopt strategies to counteract their influence. This includes improving self-awareness, seeking a variety of perspectives, and employing tools to regulate emotional reactions to market shifts. Policymakers can further contribute by fostering market transparency and educating investors, which can diminish the impact of psychological biases on investment choices. In conclusion, a concerted effort to comprehend and tackle investor psychology is essential for cultivating a more stable and efficient investment landscape.