From the dual perspective of product portfolio breadth and depth， this study reveals the double-edged sword effect of product diversification in regional public brands of agricultural products on consumer brand attitude through four sets of experiments. The findings indicate that the breadth of a regional public brand’s product portfolio reduces search costs but diminishes perceived expertise， thereby lowering overall consumer brand attitudes. Similarly， the depth of the product portfolio enhances preference satisfaction but leads to choice overload， which does not significantly improve consumer brand attitude. Therefore， regional public brands of agricultural products are not suitable for product diversification. However， boundary conditions matter： as the administrative level of the brand increases， the negative mediating effect of perceived expertise significantly weakens； when product information is provided， the negative mediating effect of choice overload becomes insignificant. Therefore， it is recommended that regional public brands of agricultural products prioritize building around specific categories， avoid covering too many similar varieties， and cautiously advance product diversification by taking boundary conditions into account.