The present disclosure relates to a method for preparing a thermochromic water-based gel ink composition, as well as a thermochromic water-based gel ink composition thereof comprising a specific monocarboxylic or polycarboxylic acid. The present disclosure also concerns a writing instrument, such as a ballpoint pen, a brush pen, a marker, a highlighter, and a felt pen, comprising a thermochromic water-based gel ink composition according to the disclosure.