The present invention relates to a preparing method of bismuth sodium titanate-barium titanate ferroelectric single crystal, belonging to the field of crystal growth. The preparing method provided by the invention comprises a series of single crystal growth processes: raw material treatment, heating and melting, growing and cooling. The rotational speed of a seed rod is 10 to 50rpm, and the drawing rate is 0.1mm to 20mm per day, and the cooling rate is 1 to 20 DEG C per day; the cooling rate is 10 to 150 DEG C per hour after the crystal finishes growing. The technology provided by the invention can grow bismuth sodium titanate-barium titanate ferroelectric single crystal (NBBT) with the diameter larger than 40mm and the length larger than 10mm, and the piezoelectric constant d33 can reach 280pC/N, and the electromechanical coupling factor kt can reach 50 percent. The material with the piezoelectric property which reaches the level of leaded piezoelectric materials of PZT ceramics, etc., can be applied in practice.