A writing instrument with a multivariate mechanical doll consists of an elongated pen body having a seat mounted on an end thereof, a working piece and a doll. The action of the doll is in response to the mechanism of the working piece which is driven by extending and retracting the refill within the pen body. The doll which composes of a frontal and a rear casing incorporates a decorating object pin jointed to the working piece. Said seat includes a plate which attaches to the rear casing extended upwardly from a top thereof; and a rail is provided at a frontal portion of said plate and a locating post is provided at a rear portion thereof. The locating post is sheathed by a spring and engages to a male junction piece. Said junction piece connects to a female junction piece which fastened onto the frontal casing and engaged with a third driving element of the working piece which has a waving surface. The waving surface exerts and lifts the frontal casing allowing the decorating object to expose. The frontal casing can be replaced without any tool required.