The invention relates to malic dehydrogenase A which comes from corbrin-capsule-producing strain cordyceps sinensis hirsutella sinensis, participates tricarboxylic acid cycle and is used for citric acid synthesis, a gene encoding the malic dehydrogenase A, and application of the above two. The amino acid sequence of the malic dehydrogenase A and protein shown as SEQ ID No. 1 have 90% or more of homology, and the nucleotide sequence of the encoding gene and a sequence shown as SEQ ID No. 2 have 90% or more of homology. A cloned DNA of the provided nucleotide sequence can be transferred into an engineering bacterium through a transduction, transformation or conjugal transferring method. By adjusting expression of the encoding gene corresponding to the enzyme catalyzing malic acid for preparing corresponding oxobutanedioic acid, a host is endowed with property of highly expressing malic dehydrogenase A, an effective approach is provided for expanding biological application of malic dehydrogenase A, and important application prospect is provided.