Manufacturing method of BiCMOS integrated circuit

The present invention provides a manufacturing method of a BiCMOS integrated circuit. The method comprises a step of orderly forming a first oxide layer and the silicon nitride layer on the surface of a substrate, a step of removing the silicon nitride layer in a preset first area, forming second oxide layers in the first area, wherein the edges of the second oxide layers are in the shape of a beak with increasing thickness, and the beak extends to the area surface outside the first area, a step of removing the remaining silicon nitride layer and forming ion implanting area in the second area surface between the second oxide layers, a step of carrying out annealing process such that the ion implanting area is allowed to be in thermal diffusion to form a base area which surrounds the beaks of the edges of the second oxide layers. Through the scheme provided by the invention, the base area can laterally spread to an area under the beaks and surrounds the beaks, when a transistor is under high voltage, a wide depletion layer can be realized in the base area, thus the reverse voltage withstanding of the BiCMOS integrated circuit is improved, the highest working voltage of the transistor is raised, and the requirement of a high voltage device is satisfied.