I cannot get the inner value of a JSON object

Yeah your response looks really wrong, a string within a JSON and all. But even if that is the expected response, Karate can handle it. Refer the documentation on type conversions: https://github.com/intuit/karate#type-conversion

* def response =
"""
[
  {
    "code": 200,
    "result": "[{\"distinct\":false,\"operatetime\":\"2019-05-17 17:01:01\",\"personid\":\"e8edec61-fd1a-4c69-8b60-fb8d21d06095\",\"sampleid\":\"1c9410cd-608d-4eb1-8d12-c8f2faf7fca4\"}]"
  }
]
"""
* json result = response[0].result
* def personId = result[0].personid
* match personId == 'e8edec61-fd1a-4c69-8b60-fb8d21d06095'