Since the 2008 international financial crisis, many countries have returned to the discretionary paradigm in macroeconomic regulation. China’s 2017 Government Work Report also stressed to strengthen discretionary regulation of monetary policies for the first time. But up to now, scholars have not reached a consensus about which one is better between discretionary and ex-ante commitment paradigms. Most of the researches support the viewpoint that playing by the rule can improve the credibility of monetary policies with containing ex-ante commitment mechanism. However, the flexibility and pertinence of discretionary regulation undoubtedly play an important role in making the economy out of recession. So this paper aims to quantitatively analyze the economic stabilization effects of discretionary and ex-ante commitment paradigms by a new Keynesian model.　　Most of the literature uses the new Keynesian framework to make an analysis based on the rational expectation hypothesis. But in China, people have obviously idiosyncratic expectations and the rational degree of expectations needs to improve. So the monetary policy analysis based on the rational expectation hypothesis is not applicable to current reality. The 2016 Central Economic Working Conference firstly put forward " maintaining prudential and neutral monetary policies”, but most of the monetary policy literature omits the impact of interest rate adjustment, so that the conclusions couldn’t adapt to the value orientation of current monetary policies. To accurately understand the efficiency of discretionary paradigms in the current Chinese context, this paper builds a new Keynesian model including idiosyncratic expectations and induces the formulas for discretionary and ex-ante commitment paradigms based on the triple-target loss welfare function with interest rate stabilization by the central bank.　　It shows that: （1） The demand shock mainly derives Chinese economic fluctuations, idiosyncratic expectations consist of rational ingredient and adaptive ingredient, and monetary policies incline to discretion. （2） The demand shock leads to a more sluggish economic fluctuation convergence than the supply shock, improving the rational degree of expectations helps to promote the whole welfare, and ex-ante commitment is not necessarily better than discretion, specifically when idiosyncratic expectations exists. （3） Monetary policies without interest rate stabilization operation result in a greater welfare loss, and the loss is greater under the demand shock than under the supply shock. 　　It suggests that: （1） Discretion is reasonable under idiosyncratic expectations, and monetary policies shouldn’t be blindly transformed into the ex-ante commitment paradigm. （2） Monetary policies must be focused on interest rate stabilization under the condition of economic fluctuations mainly driven by the demand shock, and prudential and neutral monetary policies can prevent unnecessary interest disturbance from affecting financial stability, so as to promote large reduction in the welfare loss. （3） In the long run, improving public expectations, gradually getting rid of external dependence and vigorously pushing the supply-side structural reform can improve the stabilization effects of monetary policy regulation.