In this paper, the surplus process is assumed to be a periodic risk model and the insurer is allowed to invest in multiple risky assets described by the Black-Scholes market model. Under shortselling prohibition, the authors consider the optimal investment from an insurer’s point of view by maximizing the adjustment coefficient and the expected exponential utility of wealth at one period, respectively. It is shown that the optimal strategies of both of optimization problems are to invest a fixed amount of money in each risky asset.