The invention discloses a novel element mounting beam composite structure. The composite structure is formed by combining an element mounting beam I, an element mounting beam II and a basic mounting beam. For element mounting pitch reference dimension of 1/5E step pitches, any data among the 1/5E step pitches can be realized by a front reference dimension and a rear reference dimension so as to greatly improve the practicability of the design. For all the continuous dimensions of twice equal pitches and more than twice pitches, the structure can be combined to avoid the unnecessary repeated design of the mounting pitches of the element mounting beam and can acquire any accurate mounting dimension of the element by selecting one combination. The element mounting beam composite structure has the characteristics of simple structure, reasonable design, low production cost, convenient and fast use, accurate and high-efficient positioning, high consistency and high interchangeability.