It is well know that prey-predator interaction is one of the fundamental interspecies interactions in ecology. A predator-prey interaction has been described firstly by Lotka and Volterra in two independent works, naming Lotka-Volterra prey-predator system, one of the fundamental population models in theoretical biology. After them, prey-predator interaction has been studied systematically in past decades. Many works on prey-predator interaction have mainly focused on the negative effect produced by predation, but no attention on the positive effect produced by predation. A set of theoretical models of co-evolution and genetic feedback mechanism suggested by laboratory experiments have shown that predation can produce the positive effect for prey. Dawkons & Krebs's description of predator-prey coevolution as an arms race also helped to revive interest in the field. Considering the positive effect for prey during the predation, this paper introduces the positive effect for prey into the classical predator-prey model and a new predator-prey system is constructed. By dynamics analysis of the system, the condition for the existence of the positive equilibrium is given. The stability of the equilibriums are also discussed, and the positive equilibrium is either stable or unstable and either a node or a focus depending on the parameter conditions. Especially we show our model has an unstable limit cycle by using the Hopf bifurcation theorem. By investigating the influence of positive effect for prey on the dynamics of the coexistence equilibrium in the first quadrant, the result show, (1) the suitable positive effect for prey in the classical predator-prey model can strengthen the stability of the system, (2) there is an unstable limit cycle in the system when increasing the positive effect reaching a certain level, and (3) the stability of the system will be changed, and both the predator and prey go towards the infinite as we continue to increase the positive effect. The computer simulation of the system supports our main results and illustrates them intuitively. Through computer, the phase portraits of all kinds of equilibriums are drawn, such as a saddle, a stable node, a stable focus, an unstable focus and an unstable limit cycle. The predictions of the model are emphasized by the results of simulation. Combining the phase portraits with the consequence predicted by simulation, the model we established can be easily to understand. These predictions of the model have the vital significance in conservation biology. From the odocoielus nemionus case which lived in Arizona Kaibab grassland of United States, we conclude that the increase in the defensive ability of prey is to increase the positive effect for prey in our system. First, the increase of positive effect has strengthen the stability of the system, while more increase in the defensive ability of prey would lead to the change of the system stability, finally it made both the predator and prey go towards the infinite and our system collapsed. It is why the number of odocoielus nemionus reduced with the reduction of their natural enemies. The case is consistent with the prediction of our model, and our model give a more reasonably explanation for the odocoielus nemionus case. On the other hand, the case has shown that our model is more realistic and reasonable.