The invention relates to a flushing liquid for oil-based drilling fluid well cementation and a preparation method thereof. The preparation method of the flushing liquid comprises: adding 1 part by weight of a suspending agent into 70 parts by weight of fresh water, stirring for 3 min with a speed of 4000 rpm, and dissolving for 30 min; adding 1-3 parts by weight of a chelating agent and 1-6 parts by weight of a cleaning auxiliary agent, and stirring for 10 min with a speed of 4000 rpm; with the stirring speed of 4000 rpm, adding 20 parts by weight of solid particles within 20 s, and stirring for 1 min; weighing 30 parts by weight of 40 DEG C fresh water, dissolving 5-15 parts by weight of a cleaning agent into the water; and finally uniformly mixing the cleaning agent solution with the above prepared solution, so as to prepare the flushing liquid. The flushing liquid has the characteristics of relatively strong permeation effect, oil flushing property and the like, used to flush oil slurry and oil films attached to well wall and casing pipe wall, helps to improve the affinity of a drilling fluid with a cement sheath and enhance the cementation property between the cement sheath and well cementation first and second interfaces, has the compatibility with conventional cement paste and oil-based drilling fluids, and is capable of improving and enhancing the rheological property of a drilling fluid system.