We identify the topological nodal-line semimetal (NLSM) $\mathrm{Ca}\mathrm{Ag}\mathrm{As}$ as an efficient electrocatalyst for the nitric oxide reduction reaction, offering an alternative to the energy- and carbon-intensive Haber-Bosch process. First-principles calculations reveal that $\mathrm{Ca}\mathrm{Ag}\mathrm{As}$ hosts a nodal loop around the Fermi level, generating drumheadlike topological surface states (TSSs) protected by mirror symmetry. These TSSs provide a high surface density of states (SDOS) near the Fermi level, which underpins excellent catalytic activity of $\mathrm{Ca}\mathrm{Ag}\mathrm{As}$. The Gibbs free energy ($\mathrm{\Delta }G$) of the potential-determining step is 0.22 eV, lower than that of conventional $\mathrm{Cu}$ catalysts, while high ammonia selectivity is ensured by larger free-energy increases required for byproduct formation ($>1.48$ eV) and suppression of the hydrogen evolution reaction. External perturbations, such as strain and doping, are applied to $\mathrm{Ca}\mathrm{Ag}\mathrm{As}$, revealing a quantitative linear correlation between the SDOS and $\mathrm{\Delta }G$, thereby highlighting the role of TSSs. Strain engineering can further reduce $\mathrm{\Delta }G$ to 0.08 eV by tuning the nodal-loop size. Symmetry-breaking strategies confirm that the catalytic performance deteriorates when TSSs are destroyed, as $\mathrm{Ca}\mathrm{Ag}\mathrm{As}$ transitions to a Weyl semimetal or a trivial insulator. Overall, this work uncovers an application of NLSMs in electrocatalysis and provides a topological design strategy for eco-friendly ammonia synthesis.

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• Chemical kinetics, dynamics & catalysis
• Chemical reactions
• Electrocatalysis
• Electrochemistry
• Energy-efficient industry
• Reactions at surfaces & chemisorption
• Nodal-line semimetals
• Density functional calculations