The invention discloses a high temperature resistant rheological modifier for a solid-free drilling fluid and a preparation method of the high temperature resistant rheological modifier. According to the rheological modifier, three monomers with different functional groups are polymerized under the condition of a redox system through the reasonable molecular design, molecular chains of a correspondingly generated polymer is fully extended and mutually act in the space to form a space net structure, other assistants mutually matched with the polymer are also used, so that the prepared rheological modifier has the shearing dilutability similar to that of a biopolymer and effectively resists high temperature and salinity, the highest tolerable temperature is up to 170 DEG C, and the rheological modifier is an efficient rheological modifier suitable for fresh water and saline water solid-free drilling fluid system; in addition, the preparation method of the rheological modifier is simple and feasible, the cost is also lower than that of the foreign like product, and the rheological modifier is quite favorable for market popularization and application.