The influence of directionally solidified process on porosity in cross-section transitions of dummy blade nickel-based superalloy DZ417G was investigated. It was found that the content of porosity at transition platform near the central runner was an order of magnitude higher than it is near the heater. In a certain casting temperature range (1480~1580 ℃)，with the increase of casting temperature，the content of porosity at transition platform near the central runner increased at first then decreased，the temperature with maximum content of porosity was 1550℃，however，in this casting temperature range，with the increase of casting temperature，the content of porosity at transition platform near the heater decrease a little bit. In a certain withdrawal rate (1~10mm/min)，with the increase of withdrawal rate，porosity at transition platform near the central runner increases gradually，however porosity at the opposite side part decreases. Based on these results，the relationship between casting temperature withdrawal rate and the content of porosity in transition platform dummy blade has been proposed. Besides，the influence of directional solidification process on the formation of porosity on the cross-section of the platform is mainly caused by the variation of temperature gradient of the solid-liquid interface.