Two difficult problems arise in preparation of large-diameter (< = 40mm) HgCdTe crystal ,i. e. high mercury pres-sure leading to explosion of growth ampoule and serious composition segregation leading to compositional unhomogeneity.These problems are analysed in the paper ,and the solutions for these problems are found. The first is to exert suitable pres-sure outside the ampoule to balance the part interior mercury pressure of it. The second is to grow higher composition HgCdTe crystal from HgCdTe melt with lower composition at a lower temperature which can reduce the mercury pressure and decrease the compositional segregation. The final is to add higher composition HgCdTe into lower composition HgCdTe melt to compensate consumed CdTe.