Since 1998, China began to carry out the market-oriented reform of housing supply, housing prices in various cities soared, caused widespread concern of the whole society, we are particularly concerned about whether there is a bubble? Based on the condition of no arbitrage between purchase and rent, this paper deduces a formula to calculate the reasonable value of the purchase and rent ratio, and then calculates the value by combining with the real economic data. After that, we use the collected data of actual housing transactions in Beijing and Tianjin to estimate the purchase-to-rent ratio at the street level (community). By comparing the reasonable value, we estimate that the bubble degree of the real estate market in Beijing and Tianjin is 35.0% and 41.3% respectively.