The invention relates to a steel dam space truss structure unit body and an arch-style steel dam. The unit body is a hexahedral space truss structural body with a front end surface and a rear end surface in key pattern structures and other lateral surfaces in a stud link chain structure, connecting rods are respectively arranged between a central node O of the front end surface and other nodes, supporting connecting rods are respectively arranged between four middle nodes A,B,C and D of the front end surface and four vertexes of the rear end surface, and supporting connecting rods are respectively arranged between a central node O' of the rear end surface and the four vertexes of the rear end surface. The steel dam, which is a space truss structure with a curve upstream surface, is formedby combining the space truss structure unit bodies, an eliminator is fixed on the curve upstream surface, the bam bottom of the steel dam is fixed with a reinforced concrete bearing platform, and both sides of the steel dam are respectively fixed with the stepped bearing platform formed by reinforced concrete structures at both banks. The steel dam is kept stable mainly by the counteraction of arc ends at both banks and has strong overload capacity, high degree of safety and engineering investment saving.