The invention provides a method for determining stability of small interference of a direct-current power grid. A main circuit of a modular multi-level converter is equivalent to a simplified equivalent model; a converter electromagnetic transient model and a converter controller system are established and then are processed by linearization to obtain an active power small interference basic mathematical model; a small interference linearization Thevenin equivalent model of a converter and a small interference linearization generalized mathematical model of a direct-current power grid are established; and a small interference stability determination basis of the direct-current power grid is obtained. According to the determination method, the architecture is clear and simple; the method is easy to understand; the internal commutation process of the converter is simplified; a mathematic model can be established and analyzed conveniently. A small interference linearization model of a converter can be popularized and restriction of the original equivalent model is broken through, so that the method is suitable for all work conditions; and the direct current oscillation can be suppressed and a stability analysis can be realized conveniently. And a reliable and accurate basis for studying the mutual influence of alternating-current and direct-current power grids is provided, so that the operation reliability of the alternating-current and direct-current power grids can be improved.