The invention provides a preparation method for a flash memory. A groove disposed at an isolation structure between floating gates by use of wet etching twice and combination with a dry etching method in the middle. Compared to a conventional mode of adopting a pure wet etching method, a silicon oxide barrier layer for protecting the floating gates is formed before the added drying etching, and therefore, when the dying etching of anisotropy etching is performed, under the condition that it is ensured that the floating gates are not damaged, the groove whose cross section is an inverted trapezoid, at the time when the shortest distance between an active region and a control gate is increased, the consistency of the distance is ensured, the capacitance area between the control gate and the floating gate of the flash memory is increased, the gate coupling coefficient is improved, the rated leakage currents of the memory are enhanced, and accordingly, the erasing speed of the flash memory and the device reliability during cycling operation are improved; and when the isolation structure and the silicon oxide barrier layer are both made of silicon oxide, after the dry etching, what is needed is only to carry out etching by use of the same solution, the process steps are reduced, and the cost is decreased.