In the design of prestressed beams in building structures,the prediction of the number of prestreesed tendons is necessary.The tendon number meeting the demands of crack resistance was studied.The formulas for calculating the number of prestressed tendons under three crack resistance grades were listed.By introducing reasonable assumptions,the simplified calculation formulas for determining the tendon number at the mid-span and support point were provided.The various influential factors of the tendon number,e.g.,the beam section shape,span of the beam,section width,crack resistance grade,slab thickness and position of prestressed tendons were discussed respectively.The relationship curves between the number and various influenctial factors were presented.The impact extent of various factors on the number was compared.A relatively accurate basis for estimating the number of prestressed tendons in the primary design phase was provided for structural designers.