Mean-Variance Model An investor invests a given sum of money in a certain period of time. At the beginning of the period, he bought some securities and then sold them at the end of the period. Then at the beginning, he has to decide which securities to buy and how to allocate funds on these securities, that is to say, investors need to choose an optimal combination from all possible securities combinations at the beginning. At this time, investors have two decision-making goals: as high a rate of return as possible and as low an uncertain risk as possible. The best goal should be to achieve the best balance between these two mutually restrictive goals. The investment model thus established is the mean-variance model.