What you see here is a rare type of jian (d.e. straight blade) fitted up as an executioner's sword. I have seen only one other photo of this type, the sword in that pic is of similar dimensions but the edges are radiused, not angular, where they meet the tip. This other photo is even more macabre, it shows the headsman squatting beside a number of heads, holding the sword up in plain view for the camera. There was no caption or annotation on that pic indicating the date or what area of China it was taken.
As in Vietnam, there was no one characteristic type of "beheading sword". In the military, a regulation-style liuyedao often served the purpose. (you might want to refer to T. T. Meadows, "Description of an Execution at Canton", in the JOURNAL OF THE ROYAL ASIATIC SOCIETY, Vol 16 No. 58 (1856). Otherwise, the common two-handed falchion "dadao" popular with militia and civilian martial artists of the lower classes was widely used. Over 20 years ago, an old Navy vet stationed on the Yangtze in the 1930s told me about a beheading that he saw on the streets of Shanghai (Goumindang constables dispatching a drug peddler), and his clear recollection described a classic specimen of a niuweidao, or ox-tail saber. The old salt was quite impressed with the "daofa" of the guy wielding the saber!