In recent years, with the expansion of the offshore RMB market in Hong Kong, the relation of the offshore and onshore markets becomes more closely. However, with the currency being not fully convertible, and capital project being not completely opened, developing the market of RMB in Hong Kong will adversely affect the implementation of monetary policy. Firstly, this paper analyzes the mechanism of the offshore RMB market to the domestic monetary policy in two parts of interest rates and exchange rate respectively. Then we used the relevant variables to establish a VAR model and made the Granger Causality tests and impulse response to analyze the relationship of the offshore and onshore markets with the daily data from July 1st, 2013 to August 1st, 2015. The empirical results show that the interest rates of mainland market will play a leading role in the rates interest of Hong Kong. The spot exchange rate of USD/RMB in Hong Kong will infect the rate in mainland each other.