As an important and promising experimental method for simulating the containerless state in outer space, acoustic levitation provides excellent contact-free condition to investigate solidification process. Meanwhile, the radiation pressure and acoustic streaming caused by nonlinear effects bring various kinds of novel phenomena to crystallization kinetics. In this work, high-speed CCD, low-speed camera and infrared thermal imager were used simultaneously to observe the crystallization process of acoustically levitated SCN-DC transparent alloys. The undercooling ability and solidification process of alloy droplets with different aspect ratios were explored under acoustic levitation state. For hypoeutectic SCN-10wt%DC, eutectic SCN-23.6wt%DC and hypereutectic SCN-40wt%DC alloys, the experimental maximum undercoolings reached 22.5(0.07T_L), 16(0.05T_E) and 32.5K(0.1T_L) respectively and the corresponding crystal growth velocities were 27.91, 0.21 and 0.45 mm/s. In SCN-10wt%DC hypoeutectic alloy, the nucleation mode of SCN dendrite changed from edge nucleation to random nucleation with the increase of undercooling. For SCN-23.6wt%DC eutectic alloy, when undercooling exceeded 12.6K, DC dendrite preferentially nucleated and grew, and then the (SCN+DC) eutectic grew attached to DC dendrite. Moreover, the growth interface of DC dendrite gradually changed from sharp to smooth within SCN-40wt%DC hypereutectic alloy as the undercooling degree rose. The undercooling distribution curve and nucleation probability variation trend were analyzed versus aspect ratio. It was found that as the aspect ratio increased, undercooling of alloy droplet increased firstly, then decreased, and finally remained almost unchanged. Further analysis showed that with the increase of aspect ratio, the cooling rate would rise and thus enhanced the undercooling. However, the increase in surface nucleation rate and the droplet oscillation inhibited deep undercooling of alloy droplet. Therefore, the coupled effects of cooling rate, surface nucleation rate and droplet oscillation determined the undercooling of the alloy. In the case of SCN-40wt%DC hypereutectic alloy, the acoustic streaming and surface oscillation arising from acoustic field were the principal factors intensifying surface nucleation.