Starting from a tight binding description,the existence of mixed s-dwave pairing state is discussed. Then,within the framework of the phenomenological G-L theory, the structure of vortex lattice in a high-Tc superconductor is studied.It is shown that there is a strong correlation between the structure of a single vortex and the shape of the vortex lattice.At low temperatures,with the existence of s-wave component and its coupling to d-wave component,the d-wave order parameter and the local magnetic field show tetragonal anisotropy,and the structure of vortex lattice is oblique;however,when the temperature is close to Tc, it may become triangular.