A stable solution of a ring-opened polymer is provided. The stable solution can be prepared using the following steps: (d) adding an N,O-heterocyclic compound of formula I or II and a DOPO-based compound into an organic solvent to provide a reaction solution: wherein R1 to R3, W1, W2, m, n, p and q are as defined in the specification. (e) heating the reaction solution to carry out a ring-opening reaction to provide a polymer solution; and (f) cooling the polymer solution to obtain the stable solution, wherein the organic solvent is unreactive to the N,O-heterocyclic compound and the DOPO-based compound, and the amount of the DOPO-based compound is about 0.01 wt % to about 50 wt % based on the total weight of the N,O-heterocyclic compound and the DOPO-based compound. The stable solution is useful as a self-crosslinkable resin or a hardener for hardening epoxies.