Aulnois-sous-Laon is located some 30 km south-east of Saint-Quentin and 5 km due north of Laon. The A26 autoroute passes through the north-eastern part of the commune but has no exit in the commune. The D967 road also passes through the commune from Crecy-sur-Serre in the north continuing to Laon in the south. The village can be accessed by the D541 branching west from the D967 passing through the village and continuing west to Vivaise. The D545 road also come from the south through the village and continues north-east to Barenton-Bugny. The commune is predominantly farmland with a little forest.
The Ru d'Alnois rises south of the village and flows north-east changing to the Ruisseau de Longuedeau and forming the north-eastern border of the commune before continuing north-east joining the Ru des Barentons which eventually joins the Souche river at Barenton-sur-Serre.
The official status of the blazon remains to be determined
Blazon: Or, with escutcheon gules charged with three martlets in sable supporting a castle of four towers argent port and windows sable all supported by two Alder branches fructed in vert and posed in inverted chevron, all accompanied by three martlets sable at dexter chief, sinister chief, and base.
In 2010 the commune had 1,292 inhabitants. The evolution of the number of inhabitants is known through the population censuses conducted in the town since 1793. From the 21st century, a census of municipalities with fewer than 10,000 inhabitants is held every five years, unlike larger towns that have a sample survey every year.[Note 1]
^At the beginning of the 21st century, the methods of identification have been modified by law No. 2002-276 of 27 February 2002 , the so-called "law of local democracy" and in particular Title V "census operations" which allow, after a transitional period running from 2004 to 2008, the annual publication of the legal population of the different French administrative districts. For municipalities with a population greater than 10,000 inhabitants, a sample survey is conducted annually, the entire territory of these municipalities is taken into account at the end of the period of five years. The first "legal population" after 1999 under this new law came into force in 1 January 2009 and was based on the census of 2006.