Ninja forms: Ajax submit only return a json string

I use Ninja forms and select the “submit via ajax” option.However, when I click the submit button, it returns a new page with the json string:

{"form_id":1,"errors":false,"success":{"success_msg":"\u003Cp\u003EYou form has been successfully submitted.\u003C\/p\u003E\n"},"fields":{"1":"aa","2":"aa@aa.com","3":"aaaa"},"form_settings":{"form_url":"http:\/\/localhost:81\/t4t\/wp-admin\/admin-ajax.php?action=ninja_forms_ajax_submit","form_title":"Contact Form","show_title":"0","save_subs":"1","logged_in":"0","append_page":"","ajax":"1","landing_page":"","clear_complete":"1","hide_complete":"1","success_msg":"\u003Cp\u003EYou form has been successfully submitted.\u003C\/p\u003E\n","email_from":"shaojz@gmail.com","email_type":"html","user_subject":"Thank you for contacting us!","user_email_msg":"\u003Cp\u003EThank you so much for contacting us. We will get back to you shortly.\u003C\/p\u003E\n","user_email_fields":"0","admin_mailto":"","admin_subject":"","admin_email_msg":"\u003Cbr\u003E\u003Cbr\u003EUser Submitted Values:\u003Ctable\u003E\u003Ctr\u003E\u003Ctd width='50%'\u003EName:\u003C\/td\u003E\u003Ctd width='50%'\u003Eaa\u003C\/td\u003E\u003C\/tr\u003E\u003Ctr\u003E\u003Ctd width='50%'\u003EEmail:\u003C\/td\u003E\u003Ctd width='50%'\u003Eaa@aa.com\u003C\/td\u003E\u003C\/tr\u003E\u003Ctr\u003E\u003Ctd width='50%'\u003EMessage:\u003C\/td\u003E\u003Ctd width='50%'\u003Eaaaa\u003C\/td\u003E\u003C\/tr\u003E\u003C\/table\u003E","admin_email_fields":"1","admin_attach_csv":"0","email_from_name":"T4T\u5de5\u4f5c\u5ba4","sub_id":6,"admin_attachments":[],"user_attachments":[],"save_msg":"","processing_complete":1},"extras":{"_ninja_forms_display_submit":"1","_form_id":"1","_ninja_forms_field_5":"Send"}}

Why? How can I fix this?
http://wordpress.org/plugins/ninja-forms/