The invention discloses a penicillin G sulfoxide dimethyl formamide (DMF) composite crystal and a preparation method thereof. The melting point of the composite crystal is not lower than 101 Celsius degrees. The element contents are that 53.6% of carbon (C), 9.9% of nitrogen (N), 22.8% of oxygen (O), 5.9% of hydrogen (H) and 7.55% of sulfur (S). The preparation method is that firstly, the penicillin G sulfoxide is dissolved in the DMF water solution, the water solution containing the DMF or organic solution containing the DMF to obtain the DMF mixed solution of the penicillin G sulfoxide, and then the DMF mixed solution of the penicillin G sulfoxide is cooled and placed quietly, the penicillin G sulfoxide DMF composite crystal is separated out, and the penicillin G sulfoxide DMF composite crystal is filtered, washed and dried. The penicillin G sulfoxide DMF composite crystal is good in stability, low in water content and does not contain impurities to damage protection for the esterification protective reaction. The process of the esterification protective reaction and a ring-enlargement rearrangement reaction is ensured in process of producing cephalosporin alkyl acid. The cephalosporin alkyl acid obtained from the ring-enlargement rearrangement reaction is high in yield and good in quality.