Abstract: The mechanical behaviour of the end cracking during one-side SAW was com-puter-simulated by thermal elastic analysis with introduction of dummy element.Expe-riments on small size specimen and work site large size specimen have been carried out also.The rotational deformation,the mechanical mechanism of formation of the end cracking and factors affecting on it were investigated.It was concluded that for preventing the formation of the end cracking,the end opening displacement and its rate should be reduced as far as possible.Theoretical analysis and work site large size specimen experiments show that the end cracking can be effectively prevented only if the arrangement of tack welds over weld end is appropriate,and the stiffness and restia-int of slit tab plate are moderate.By the aid of the above mentioned study,the end cracking was successfully avoided during ever 130 work site actual size specimens of 12,14,16,18 and 20 mm in thickness (weld length more than 1000m)were wel-ded.