The present invention relates to a seat airbag for a vehicle in which a cushion storage space 810 capable of accommodating a front cushion 200 is provided at a front end 720 connected to the front cushion 200 in a side cushion 100, and the front cushion 200 folded in a bellows type is inserted and accommodated into the cushion storage space 810. The side cushion 100 and the front cushion 200 can be deployed sequentially with a time difference, so that the reliability of the cushion deployment can be secured.