The invention discloses a straight-through method for producing 7-amino-3-desacetoxy-cephalosporanic acid, comprising oxidation and ring enlargement. The invention is characterized by pressing a ring enlarging solution in the ring enlargement process into a hydrolyzing kettle when the ring enlarging solution is still hot; adding water which is equal to 80 percent of the total volume of the ring enlarging solution, simultaneously adjusting the pH value of the solution by 20 percent ammonia to 8.5, and keeping the adjusted solution stand after the adjusted solution is stirred and hydrolyzed to separate an aqueous phase; adding water which is equal to 40 percent of the total volume of the ring enlarging solution once again into the solution, keeping the solution stand after the solution is mixed and hydrolyzed to separate an aqueous phase, and sending an ortho-toluene organic phase in a ring enlargement system to a treatment position; adding an extracting agent into the aqueous phases which are separated twice according to a volume ratio of 1 to 0.8, and stirring and keeping the solution stand to separate an aqueous phase; adding the extracting agent which is equal to 30 percent of the volume of the aqueous phase into the aqueous phase twice, stirring and keeping the solution stand to separate an aqueous phase; stirring and decoloring the aqueous phase which is added with active carbon which is equal to 1 percent the aqueous phase by weight; and pressing the solution into a cracking kettle through a filter for cracking. The method has the advantages of simple process, small wasterwater discharge and so on.