Abstract: The pattern of crack propagation and the forced f eatures of concrete and section steel are analyzed in detail by the experiment o f the steel encased light aggregate concrete beams.The failure mode and shear me chanism have been researched.The influence factors on the ultimate shear strengt h is determined by the theory and experiment.The theoretical calculating formula s of shear strength of steel encased light aggregate beams are proposed by compl ex method.The formulas are verified by testing results.A good agreement is obtai ned between the calculating results and the experimental results.