The invention provides a potassium-sodium niobate monocrystal and a preparation method thereof. The potassium-sodium niobate monocrystal is prepared from the following raw materials of K2CO3 (potassium carbonate), Na2CO3 (sodium carbonate) and Nb2O5 (niobium pentoxide) according to a molar ratio of 0.52:0.48:1. The preparation method comprises the following steps of weighing the raw materials, putting into an oven, and drying; according to the molar ratio, weighing the raw materials, putting into a ball mill, and performing ball milling; fetching out a ball milling product, drying, tabletting, and pre-sintering; using absolute ethyl alcohol as a medium, performing secondary ball milling, and drying; screening the dried material, pressing into a round blank, and sintering, so as to obtain monocrystal; fetching the potassium-sodium niobate monocrystal out of a ceramic matrix, so as to obtain the pure potassium-sodium niobate monocrystal. The potassium-sodium niobate monocrystal and the preparation method have the advantage that a seedless solid-phase crystal growth technique is adopted under the condition of no doping of other elements, and the ratio, sintering temperature, time and blank size of the raw materials are changed, so as to obtain the pure potassium-sodium niobate monocrystal with different sizes.