After the first COVID-19 patient was diagnosed in China, in December 2019, the market's reaction to the pandemic evolved from initial unawareness and minimal response to rapid panic among investors as the outbreak escalated. As the pandemic continued to develop, the market gradually absorbed the impact, and stock market performance experienced a process of decline from high to low points. From the perspective of behavioral finance, the development of the pandemic suppressed the stock market through both profitability and risk preferences. The initial lack of understanding of the situation further exacerbated market volatility. This paper examines the correlation between the development of the pandemic and daily stock returns from March to June 2020, as well as the mechanism through which the pandemic affected investor sentiment. It reveals a negative correlation between the growth rate of infections and daily stock returns, and how the pandemic increased investors' pessimism, influencing their trading behavior by increasing turnover rates, thereby exacerbating the negative impact on stock returns. By empirically revisiting the impact of the 2020 COVID-19 pandemic on stock market investor behavior, this study provides insights for predicting and responding to the impact of the 2019 novel coronavirus (2019-nCoV) on the stock market.