The invention provides an optimized gene of high-temperature mannase ManAHr. A gene sequence of the optimized gene is shown as SEQ ID NO.2. The optimized gene can be efficiently expressed in a pichia pastoris expression system, thus the high-temperature mannase ManAHr is prepared. The high-temperature mannase ManAHr is wide in application prospect in fields such as feed additives, health foods, papermaking, washing and medicines. The invention further provides a recombinant expression plasmid containing the optimized gene, and a pichia pastoris recombinant strain for producing the high-temperature mannase ManAHr. The content of a protein of the high-temperature mannase ManAHr secreted from the strain in a fermentation liquor reaches an electrophoresis pure grade without being nearly purified, and thus the production cost is lowered.