The present invention relates to a fatty alcohol polyoxyethylene polyoxypropylene ether carboxylate oil base emulsifying agent and a preparation method thereof, wherein problems of poor emulsion stability, no salt tolerance and no temperature tolerance of the oil base drilling fluid in the prior art are mainly solved with the present invention. The technical scheme comprises that the fatty alcohol polyoxyethylene polyoxypropylene ether carboxylate containing a molecule general formula is provided, wherein M is one ion selected from hydrogen, calcium, magnesium, barium, zinc, aluminum and iron, R is C8-C30 alkyl, m is the adduction number of the ethoxy group EO, and is an integer of 1-10, n is the adduction number of the propoxy group PO, and is an integer of 1-40, and x is an integer of 1-3. With the technical scheme, the problems in the prior art are well solved, and the fatty alcohol polyoxyethylene polyoxypropylene ether carboxylate can be used in high water sensitivity and high temperature stratums and special drilling operations such as requirements of drilling of large displacement special wells and offshore horizontal wells, and the like.