The invention relates to a composite emulsifier for an oil-based drilling fluid, and a preparation method and application thereof. The main objective of the invention is to overcome the problems of a few varieties, poor emulsification performance and poor temperature resistance of emulsifiers for the oil-based drilling fluid in the prior art. The composite emulsifier for the oil-based drilling fluid in the invention comprises a main emulsifier, which is at least one selected from structures as shown in a formula (I) described in the specification, and a coemulsifier, which is at least one selected from structures as shown in a formula (II) described in the specification, wherein the ratio of main emulsifier to coemulsifier is more than 0 and no more than 1. Thus, the above-mentioned problems are overcome; the composite emulsifier can be used for the oil-based drilling fluid and has good emulsification performance and temperature resistance; and the formed oil-based drilling fluid has good stability and temperature resistance and can meet special demands of well drilling in a high-temperature stratum with strong water sensitivity, large-displacement special well drilling, an offshore horizontal well and the like in oil fields.