The crystal structure and thermal expansion behaviors of a new tetragonal tungsten bronze (TTB) ferroelectric, Pb2K0.5Li0.5Nb5O15, were systematically investigated by selected-area electron diffraction (SAED), neutron powder diffraction, synchrotron X-ray diffraction (XRD), and high-temperature XRD. SAED and Rietveld refinement reveal that Pb2K0.5Li0.5Nb5O15 displays a commensurate superstructure of simple orthorhombic TTB structure at room temperature. The structure can be described with space group Bb2(1)m. The transition to a paraelectric phase (P4/mbm) occurs at 500 degrees C. Compared with Pb2KNb5O15 (PKN), the substitution of 0.5K(+) with small 0.5Li(+) into PKN causes the tilting of NbO6 octahedra away from the c axis with Delta theta approximate to 10 degrees and raises the Curie temperature by 40 degrees C, and the negative thermal expansion coefficient along the polar b axis increases more than 50% in the temperature range 25-500 degrees C. We present that, by introduction of Li+, the enhanced spontaneous polarization is responsible for the enhanced negative thermal expansion along the b axis, which may be caused by more Pb2+ in the pentagonal caves.