Generation of non-stabilized β-ester diazos and their applications in carboxylic acid O-H insertion reactions have been reported, which afford β-acyloxy esters in excellent yield. Varieties of aryl- and alkyl-substituted diazos are well tolerated in this insertion reaction under mild and convenient conditions. Moreover, structural modification of the natural product and molecular drug can also be achieved in this reaction. This protocol not only realizes the reaction involving unstable β-ester diazos but also provides an efficient strategy for the synthesis of β-acyloxy esters.