The present disclosure provides an underbody manufacturing method and a vehicle underbody. The method includes determining dimensions for the vehicle underbody and underbody panels, selecting the underbody panels, putting the underbody panels in a stamping die for stamping to form concave and convex parts on the underbody panels, welding the stamped underbody panels to splice the underbody in the length direction and/or the width direction, so as to make the underbody reach the determined length and width. Underbodies with various dimensions can be formed using the same stamping die, and accordingly, die cost is saved and vehicle development time is shortened, which provides convenience for diversified body designs on the whole.