The invention discloses a square steel shell battery and module for assembling a battery system. The square steel shell battery and module comprises a battery cell pole group, primary internal connection flow guide sheets, secondary internal connection flow guide sheets, leading-out terminals, a battery shell, battery covers, side supports and grids, wherein the battery cell pole group is arranged in the battery shell; the primary internal connection flow guide sheets are arranged on the two sides of the battery cell pole group; the secondary internal connection flow guide sheets are arranged on the primary internal connection flow guide sheets; center holes are formed in the middles of the secondary internal connection flow guide sheets; the leading-out terminals are welded at the center holes; the battery covers are arranged on the two sides of the battery shell in a covering manner; and the grids are arranged between the side supports. With the adoption of the square steel shell battery and module, the battery uniformity is good; a sealing element is reliable; a material is thin; the energy density is high; a safety valve is difficult to block; the vibration resistance is good; the possibility of a short circuit is low; temperature distribution is uniform; and wide popularization and application can be performed.