Why do banks provide implicit guarantee to their off-balance wealth management products (WMPs) ，even though they are not required to do so? Ｒelevant domestic literature are qualitative and lack of normative model analysis． Existing foreign literature either cannot explain the phenomena perfectly，such as why the proportion of WMPs invested in non-standard assets is significantly lower than the upper limit of regulation，or ha- ven’t considered ex post information asymmetry with WMPs，thus failing to fully explain the cause of implicit guarantee． We use costly state verification to model information asymmetry on the return of WMPs，use investment method to model information asymmetry on bank’s effort，and construct a delegated portfolio management model to analyze the mechanism of implicit guarantees． The research finds when the cost of circumventing the regulation is lower than a certain amount，it makes a profit for banks to implicitly guarantee their WMPs． Further analysis shows that using signals about return to conclude contracts is helpful to alleviate incentive problems and prevent banks from providing implicit guarantee．