Abstract: The steel truss without diagonals is applied widely in all kinds of public buildings. As a kind of small structurewith high redundant number, a simplecalculation method is hard to find for reasonable results. An approximate method tocalculate this steel truss is found by use of the principleof structural mechanics and symmetries. Accurate analysis andcalculation of examples indicate that the results of this method arereasonable and the method is also convenient for applications.