How to convert base64 imgae as HTML to word document in win forms

you can try to use "insertInlinePictureFromBase64".

Example:

var img = '' //a base64 encoded string
 
// Run a batch operation against the Word object model.
Word.run(function (context) {
 
    // Create a proxy object for the document body.
    var body = context.document.body;
 
    // Queue a command to insert the image.
    body.insertInlinePictureFromBase64(img, 'End');
 
    // Synchronize the document state by executing the queued commands,
    // and return a promise to indicate task completion.
    return context.sync().then(function () {
        app.showNotification('Image inserted successfully.');
    });
})
.catch(function (error) {
    app.showNotification("Error: " + JSON.stringify(error));
    if (error instanceof OfficeExtension.Error) {
        app.showNotification("Debug info: " + JSON.stringify(error.debugInfo));
    }
});


Reference:

Inserting a base64 encoded image into a Word 2016 document

Other references:

InlinePicture object (JavaScript API for Word)

Body object (JavaScript API for Word)