The invention belongs to the technical field of large-thickness aluminum alloy electron beam welding, and particularly relates to a large-thickness aluminum alloy electron beam welding technology lock bottom structure design method. The method is applicable to the welding of an aluminum alloy electron beam with the thickness of 150 mm-180 mm, and comprises the steps of 1, determining the shape of an air discharge duct; 2, determining the size of the air discharge duct, wherein firstly, calculating the volume of the air discharge duct and the area of the air discharge duct area per unit length, secondly, determining the angle theta 60 degrees of an isosceles trapezoid cross section, thirdly, calculating the width of the groove opening of a dovetail groove, and fourthly, determining the depth of the dovetail groove. By adopting the method of penetration welding, and meanwhile adding a technology lock bottom structure provided with a air discharge duct structure on the reverse side of a welding line innovatively, according to the large-thickness aluminum alloy electron beam welding technology lock bottom structure design method, it is guaranteed that 1, a lock bottom 'holds' welding leaking metal during penetration welding to avoid indentation on the obverse side of the welding line caused by welding leaking metal outflow; 2, the number of air discharge ducts is increased, the discharge of gas inside the welding line is accelerated, and the deposit of defects at a welding line root portion is reduced.