From the insurer’s point of view,this paper studies the optimal investment and proportional reinsurance in the Sparre Andersen model.Under the criterion of maximizing the adjustment coefficient, the authors obtain the closed form expressions of the optimal strategy and the maximal adjustment coefficient,and derive the explicit expression of the ruin probability or its lower bound when the claim sizes are exponentially distributed.Some numerical examples are presented,which show the impact of model parameters on the optimal values.It can also be seen that the optimal strategy to maximize the adjustment coefficient is sometimes equivalent to those which minimize the ruin probability. From the insurer’s point of view, this paper studies the optimal investment and proportional reinsurance in the Sparre Andersen model. UNder the criterion of maximizing the adjustment coefficient, the authors obtain the closed form expressions of the optimal strategy and the maximal adjustment coefficient, and derive the explicit expression of the ruin probability or its lower bound when the claim sizes are exponentially distributed. Some numerical examples are presented, which show the impact of model parameters on the optimal values. It can also be that that the optimal strategy to maximize the adjustment coefficient is sometimes equivalent to those which minimize the ruin probability