The embodiment of the invention provides a method and a device for determining system stability during device grid connection. The method comprises the steps of: obtaining electrical parameters of a device, and testing line conditions of device; obtaining an admittance matrix of the device and an impedance matrix of the line according to the electrical parameters and the line conditions, and calculating a return rate matrix of a network of the equipment grid connection according to the admittance matrix and the impedance matrix; and acquiring curves respectively corresponding to each diagonal element in the return rate matrix when the frequency changes on a complex plane, and if all the curves are determined to meet preset conditions, and determining that the device is in an oscillation stable state when being connected to the grid, wherein the preset condition comprises an oscillation influence factor. The device performs the above method. The method and the device for determining system stability during device grid connection can accurately determine the oscillation stability of the device during the grid connection.