Casting samples contained different forms of graphite are obtained by different nodularizing and inoculation treatments in vacuum mid-frequency induction furnace. The lower bainite cast irons are prepared after continuous cooling in nitrate and nitrite mixed media and tempering at 250 ℃ for 2 h. The influence of nodularizing and inoculation treatment on spheroidization effect is confirmed. The relationships between microstructure characteristic, mechanical property and graphite forms are investigated. Results show that：The temperature and time of spheroidizing treatment have a significant effect on the absorptivity of Mg, the evaporation loss of Mg is intensified as the temperature exceeds 1 500 ℃, and the absorptivity of Mg reaches up to 64.2% as spheroidizing treatment temperature of 1 450 ℃ is used; Meanwhile, there is also obvious influence of inoculation treatment on the form and distribution of graphite, as the addition of post inoculation increases up to 1.2%, the graphite is spheroidized adequately. The spheroidization rate of graphite is 93%, and the grain size reaches up to grade 9; In addition, the comprehensive mechanical property of lower bainite cast iron after heat treatment is improved for the small size and uniform distribution of spheroidal graphite.