The issue of securities investment portfolio has always been important because it is not only an important business activity in the financial market, it is also closely related to people's lives and involves personal financial management activities. Markowitz’s "Securities Portfolio Selection" paper is considered the starting point of modern portfolio theory. He first proposed to use the expected return and variance of securities to study the issue of how to choose the investment portfolio. The mathematical model he developed was called the Markowitz Mean-Variance Model, which was later widely used in investment activities. The Markowitz Mean Variance Model can use historical data to analyze the returns and risks of the portfolio of invested securities, helping investors to withstand low risks and obtain effective returns. Researching the practical application of the Mean Variance Model in China’s securities market in the current situation of China's public financial investment activities have certain practical significance.This article uses four stocks selected by the CSMAR database as a case, extracts the daily closing price and return data from December 14, 2020 to December 18, 2020, establishes a mean-variance model, and applies spss software to calculate the mean value, variance, standard deviation, covariance and correlation coefficient of stock portfolios and other data. Empirically analysing how investors can optimize stock investment by combining their own risk preference, acceptable average return or investment ratio under investment portfolio conditions with historically valid information such as known return averages, correlation coefficients, etc. Making the selected stock portfolio diversing investment risks and achieve the ideal goal of maximizing investment utility.The sequence of the main part of this article is as follows: First, the relevant background knowledge of the portfolio theory is introduced. Then Markowitz’s portfolio theory and its theoretical flaws and theoretical development is explained in detail, especially focusing on the basic ideas and methods of Markowitz’s mean variance model. Also there are the mathematical expression of return and risk, as well as the basic understanding of the correlation coefficient such as the correlation between covariance and covariance, the basic derivation process of the model is also covered. Then apply the basic mean-variance model to the Chinese stock market to conduct a simple empirical analysis. Four stocks are selected from the Shanghai and Shenzhen 300 Indexes in Chinese stock market, and Markowitz’s mean-variance model is used to analyze the relationship between portfolio stocks and analyze how to select and allocate them.Finally, the research concluded that the objective laws of securities investment is that returns always coexist with risks, and high returns are often accompanied by high risks. But people can use scientific model methods to achieve a balance between returns and risks and make appropriate investments choices
