The invention relates to a growth method of a fluxing agent of 
boron phosphate monocrystal, which comprises the steps of: 1) proportionally mixing a 
boron phosphate compound and the fluxing agent, putting the mixture into 
platinum crucible, and heating till completely fusing in a 
crystal growing furnace; and then cooling to the temperature being 2 to 15 DEG C above a saturation temperature, thus obtaining a high temperature melt containing 
boron phosphate and the fluxing agent; and 2) putting 
seed crystal loaded on a seed rod into the high temperature melt, keeping constant temperature for 10 to 180 minutes, reducing the temperature to saturation temperature, and rotating the seed rod at the speed of 10 to 50 revolution / minute; and reducing the temperature at the speed of 0.1 to 2 DEG C / day, lifting the 
crystal from the liquid surface after the 
crystal grows up, and cooling to 
room temperature at the speed of 20 to 50 DEG C / hour to obtain the 
boron phosphate monocrystal. The fluxing agent used by the method can reduce the 
viscosity of the high temperature melt, is beneficial to melt 
mass transportation in the process of 
crystal growth, avoids the formation of inclusions in the crystal, and can stably grow the transparent 
boron phosphate monocrystal which has cm-level size, short 
ultraviolet cutoff 
wavelength and good mechanical 
processing performance, is not easy to crack, does not absorb 
moisture, and is easy to be stored.