Why I not get value from $form_state['values']['field_form_name']
The code you are showing is wrong for two reasons:


hook_form_alter() is invoked for every form Drupal is showing; this includes Drupal core forms, as well as forms implemented by third-party modules. It is not invoked for the forms of your own module. So, you first need to check for which form the hook is being invoked, then you can access the form fields the form of your module is using.
hook_form_alter() is not invoked when the form is submitted, but before the form is rendered. So you cannot use $masothue = $form_state['values']['ma_so_thue']; there. You need to add a new submission handler from hook_form_alter(), and in that submission handler you can get the submitted values, but then you cannot alter the default value for a form field in a submission handler.