This paper studies investor sentiment with three different frequencies: daily，weekly and monthly， and applies MIDAS model to study the effects of mixed-frequency investor sentiment on stock market return and volatility． It is found that the mixed-frequency sentiment has a significant positive impact on both return and volatility and that the MIDAS model outperforms the traditional regression model． The GAＲCH-MIDAS model is used to study the effect of mixed-frequency sentiment on the stock volatility，and it is found that the mixed-frequency sentiment significantly affects the long-term volatility．