The utility model relates to the technical field of fabricated steel structure, disclose a kind of fixed structure and the connecting joint of modular steel structure, fixed structure includes at least one first connecting piece, first connecting piece includes plug-in board, connecting plate and support plate, at least one horizontal direction extension slot is provided on plug-in board, connecting plate is fixed in the top side of plug-in board;The plug-in board of each slot lower part is fixed with support plate, and support plate is in contact with steel beam
flange plate up and down.The connecting joint of modular steel structure includes fixed structure and second connecting piece, first connecting part inserts and is tightly attached on the inner wall of upper column;Second connecting part inserts and is tightly attached on the inner wall of lower column;Steel beam
flange plate inserts each slot, connecting plate is fixedly connected with second connecting piece, and steel beam
flange plate is in contact with support plate up and down.The utility model structure is simple, connection is reliable, easy to disassemble, solve the technical problem of the
load bearing capacity of weakening steel beam and steel column in the prior art hole, reduce the safety of connecting joint.