The effect of naphthalene water reducing agent on unconfined compressive strength， shear strength， deformation and water stability of soft soil were investigated. The modification effect of lime soil and cement soil by water reducing agent was observed and its microscopic mechanism was also explored by SEM. The results show that naphthalene water reducing agent at suitable mixing amount can improve mechanical properties and water stability of soft soil， it also can improve the strength of lime soil； the addition order of naphthalene water reducing agent and cement has great influence on the modification effect of cement soil. When cement is added in soft soil 6 hours earlier than naphthalene water reducing agent， the unconfined compressive strength of cement soil will be further improved， especially for the early strength. After modified by naphthalene water reducing agent， the structure of soft soil become interlaced and more compact， the hydration products is richer and the distribution is more uniform in lime soil and cement soil when modified by naphthalene water reducing agent.