Return large string from ajax call using Jquery to Web Method of ASP.NET
Most probably you have exceeded MaxJsonLength, by default it is 102400 characters and your string is bigger. You can increase this limit in web.config:
<configuration> ... <system.web.extensions> <scripting> <webServices> <jsonSerialization maxJsonLength="300000" /> </webServices> </scripting> </system.web.extensions> ...</configuration>