Dielectric and pyroelectric properties have been measured in six ferroelectric niabate single crystals with tungsten bronze structure from 15K to room temperature. The dielectric and pyroelectric results showed that, in all the three kinds of ferroelectric niobate single crystals SBN , PBN, KNSBN with TTB structure, a new phase transition occurred between 50K and 70K, which were further supported by the X-ray experiment. The crystals symmetry changes from point group 4mm to point group mm**2,and the direction of ferroelectric polar-axis changes from the c-axis of tetragonal lattice cell to the b-axis of orghogonal lattice cell as the temperature drops. The difference between high frequency dielectric constants and low frequency dielectric constants indicates that the phase transition possesses diffuse character. According to the specific heat measurement, it is confirmed that this phase transition is a higher order transition. A model of structural change has been proposed to explain this phase transition.