Abstract: Six-story 17.6-meter-span prestressed concrete frames were used for a library.As the formwork was rare for the construction of three floors,the formwork in the first floor was moved mistakenly into the third floor for its concrete cast before prestessing tendons in the first floor were tensioned.Thus,cracks appeared on the mid-span of the first-story beams,and their width was over 1.0 mm,and height extended to bottom of slab.Pretressing tendons in frame beams of first and second floors were pulled immediately after the accident.Although major cracks on the beams were closed,the width of partial cracks was still above 0.1 mm.The reason of appearance of construction cracks was analyzed.The frame structures with construction cracks were evaluated,and retrofit scheme was proposed.It can be served as reference for prevention of the same engineering accident.