The invention discloses a flexible direct-current transmission system coupled transformer tap adjusting method. The method includes that when modulation degree of a converter surpasses the upper limit, a tap is adjusted towards the voltage direction of the coupling variable valve lowering side, and if adjustment fails, capacitive reactive instructions of the converter are actively reduced; when the modulation degree of the converter is lower than the upper limit, the tap is adjusted towards the voltage direction of the coupling variable valve rising side, and if adjustment fails, the capacitive reactive instructions of the converter are actively increased. By the method, the adjustment degree of the converter can be effectively adjusted even when the tap fails or the tap position surpasses the limits, and stable operation performance of the converter can be guaranteed.