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Coalescence and flocculation are two challenges for the stability and efficacy of pesticide suspension concentrate (SC) during the production and storage. To improve the stability of 25% pymetrozine SC, three comb‐like poly(styrene‐co‐acrylic acid‐co‐polyethylene glycol methyl ether maleate) (SA‐M) with different length side chains were synthesized by free radical polymerization and characterized by Fourier transform infrared spectroscopy and H‐nuclear magnetic resonance. Surface‐active parameters, including critical aggregation concentration (critical micelle concentration), saturation surface excess concentration (Γmax), minimum area per molecule (Amin), and standard Gibbs free energy ΔGm0 values were obtained from surface tension measurement. It shows that the surface‐active property of SA‐M decreased with the side chain length. Besides, 25% pymetrozine SC prepared using SA‐M as dispersant had low zeta potential and low viscosity but high dispersibility and thermal storage stability. It is speculated that the formation of hydrogen bonds between nonionic branches and pymetrozine molecule at the expense of interruption of intermolecular hydrogen bonding between pymetrozine and water molecule, plays a key role in the stability mechanism. [ABSTRACT FROM AUTHOR]