It is an effective method to protect components from high power laser damage by using high reflective materials. The rare earth tantalates RETaO4 with high dielectric constant suggests that they may have very high reflectivity, according to the relationship between dielectric constant and reflectivity. In this paper, the crystal structure, electronic structure and optical properties of RETaO4 (RE=Y, La, Sm, Eu, Dy, Er) have been studied by first-principles calculation. The calculated lattice parameters are in good agreement with the previously reported values. With increasing the atomic number of RE (i.e., the number of 4f electrons), 4f electron shell moves from high energy region to low energy region, showing the tendency of moving from conduction band bottom to forbidden gap and then to valence band. The relationship between the electronic structures and optical properties is explored. The electron transitions between O 2p states, RE 4f states and Ta 5d states have a key effect on optical properties such as dielectric function, refractive index, absorption coefficient and reflectivity. For the series of RETaO4, the appearance of the 4f electronic states will obviously promote the improvement of reflectivity. When the 4f states appear at the middle of forbidden gap, the reflectivity reaches the maximum. The reflectivity of EuTaO4 at 1064nm is up to 93.47%, indicating that it has potential applications in the anti-laser radiation area.