The Level Set/Ghost method was introduced to dynamically simulate the weldlines in the symmetric thin mold with rectangle cylinder precisely.The physical controlling equations were discretized by the general finite difference schemes，and the 5WENO（the fifth order weighted essentially nonoscillatory）scheme was implemented for the Level Set/Ghost equations.And then weldlines were captured accurately.Moreover，the analysis was made on the pressure，temperature and velocity at different times.The results were found to agree reasonably well with the corresponding theoretical analysis and to have higher accuracy than the numerical results of prior researches.