Giant Rashba spin splitting has been found in n-HgTe/HgCdTe quantum wells with an inverted band structure by studying the beating patterns in Shubnikov-de Haas oscillations. Fast Fourier transformation, node positions, as well as numerical simulations, are used to deduce the Rashba spin-orbit splitting and result in an excellent agreement in Rashba spin splitting energy (28-36 meV).