Considering the pore pressure variation of sidewall rock, which is caused by drilling fluid filtering, and additional stress and strain which is caused by temperature difference between drilling fluid and formation rock, the theoretical model of variation of porosity, which has considered variation of pore pressure and temperature difference, is derived, and the calculation model of safe window of drilling fluid density in deep well, which has considered variation of porosity, temperature difference and pore pressure, is finally established. Application of this calculation model shows that: when temperature difference between sidewall rock and drilling fluid in deep well is definite, with increased function of drilling fluid filtering, as increased as pore pressure of sidewall rock, the caving pressure is increased, fracturing pressure is decreased, safe window of drilling fluid density is narrowing, and unfavorable for safety drilling; and when pore pressure of sidewall rock is definite, if drilling fluid lower the temperature of sidewall rock, then as increased as temperature difference, the caving pressure is decreased, fracturing pressure is increased, safe window of drilling fluid density is widen, and favorable for the safe drilling. Moreover, if drilling fluid rises the temperature of sidewall rock, then as increased as temperature difference,the caving pressure is increased, fracturing pressure is decreased, safe window of drilling fluid density is narrowing, and unfavorable for the safety drilling. In this paper, the effects of drilling fluid filtering and temperature difference between drilling fluid and formation rock on safe window of drilling fluid density are theoretically explained, which is a of significance to the present calculation model of safe window of drilling fluid density.