We report here a distinct case of strontium removal under 1 M NaOH solution by an ultrastable crystalline zirconium phosphonate framework (SZ-7) with high adsorption capacity (183 mg g⁻¹) and in-depth removal performance (K_d = 3.9 × 10⁵ mL g⁻¹), demonstrating the potential application of SZ-7 for ⁹⁰Sr removal in highly alkaline nuclear waste.