most straightforward route from calais is A26 via Reims to A5 at Troyes, A5 to A31 to A6 at Chalons, A6 to Lyon, A46 Lyon "by-pass" to A7, A7 to A9 near Avignon, A9 to the French/Spanish border, the A7 to Barcelona. Mostly on toll roads.
An alternative would be A16 to Abbeville, A28 to Rouen, A13 to Louviers, N154 to A10 above Orleans, A10 to A71 at Orleans, A71 to Clermont-Ferrand, A75 to Pezenas, D13 to the A9, A9 to the French/Spanish border. More of that route is toll-free, but includes the awesome Viaduc De Millau.
Either route could, traffic conditions willing, get you to Barcelona in 15-16 hours from Calais.