The production process of high fructose syrup includes the steps of liquefaction, saccharification and isomerization of starch . One decolorization, one high sub-exchange→finished product evaporation→F42 fructose syrup. First, the starch milk is liquefied into a liquefied solution with a DE value of 15% to 20% under the action of a-amylase. The liquefied solution is adjusted to pH and temperature, and saccharification enzyme is added for saccharification. After the saccharification is completed, the DE value is 96. % ~ 98%, and then filter out some insolubles, such as lipids, proteins, etc., and then treat with activated carbon and ion exchange resin to become purified glucose solution. After being purified, the glucose liquid can be isomerized through a reactor equipped with immobilized isomerase to obtain fructose syrup with a fructose content of about 42%. This syrup is then filtered and separated by activated carbon, and then adjusted to pH and concentrated to a concentration of 71% to obtain 42% fructose syrup.