A novel scheme using a single polarization modulator for optical code sh ift keying (CSK) data generation is proposed and experimentally demonstrated for improving the securit y of single OCDMA user.We study the 2.5Gbit/s CSK-optical code division multiple access (OCDMA) system with simulation and experiment.T he receiver sensitivity of CSK system with balanced detector is 4.1dBm more than that of OO K in sim ulation.The bit error rate (BER) of CSK system with sing le detector is 1.15×10－8,and the BER can get 4.02×10－11 with optical threshold device.We simulate the attacking by an eav esdropper with data-rate energy detector and we find that the CSK system cannot be detected by the eavesdropper.