Based on the China Household Finance Survey (CHFS) in 2017, this paper uses Probit and Tobit models to empirically study the relationship between mobile internet accessibility and risky investment decision. The findings are as follows: Firstly, mobile internet accessibility significantly improves the probability and degree of household risky financial asset investment, and by using smartphone use proportion of other households in the district or county as the instrumental variable, this relationship is still significantly valid. Secondly, the mobile internet accessibility of households in the city or the east, or of high assets, or of higher education level has a greater positive effect on participation probability in risk asset investment, while this relationship is generally the opposite on participation proportion of risk asset investment for those kinds of households. Thirdly, mechanism analysis shows that the mobile internet accessibility can promote household investment behavior on risky finacial assets via paying more attention to financial information and access more formal financial service, but the financial knowledge channel is not supported statistically.