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(1) The development of sweet potato storage roots is impacted by nitrogen (N) levels, with excessive nitrogen often impeding development. Starch synthesis enzymes such as sucrose synthase (SUS) and ADP-glucose pyrophosphorylase (AGPase) are pivotal in this context. Although the effects of excessive nitrogen on the formation of sweet potato storage roots are well documented, the specific responses of <i>IbSUSs</i> and <i>IbAGPases</i> have not been extensively reported on. (2) Pot experiments were conducted using the sweet potato cultivar “Pushu 32” at moderate (MN, 120 kg N ha⁻¹) and excessive nitrogen levels (EN, 240 kg N ha⁻¹). (3) Nine <i>IbSUS</i> and nine <i>IbAGPase</i> genes were categorized into three and two distinct subgroups based on phylogenetic analysis. Excessive nitrogen significantly (<i>p</i> < 0.05) suppressed the expression of <i>IbAGPL1</i>, <i>IbAGPL2</i>, <i>IbAGPL4</i>, <i>IbAGPL5</i>, <i>IbAGPL6</i>, <i>IbAGPS1</i>, and <i>IbAGPS2</i> in fibrous roots and <i>IbSUS2</i>, <i>IbSUS6</i>, <i>IbSUS7</i>, <i>IbSUS8</i>, <i>IbSUS9</i>, <i>IbAGPL2</i>, and <i>IbAGPL4</i> in storage roots, and then significantly (<i>p</i> < 0.05) decreased the SUS and AGPase activities and starch content of fibrous root and storage root, ultimately reducing the storage root formation of sweet potato. Excessive nitrogen extremely significantly (<i>p</i> < 0.01) enhanced the expression of <i>IbAGPL3</i>, which was strongly negatively correlated with the number and weight of storage roots per plant. (4) <i>IbAGPL3</i> may be a key gene in the response to excessive nitrogen stress and modifying starch synthesis in sweet potato.