We report observations made with the IRAM 30 m radiotelescope in the HCN(1-0) and HCO + (1-0) lines towards a sample of molecular complexes (GMCs) in the disk of the Andromeda galaxy (M 31). The targets were identified bright CO GMCs selected from the IRAM 30 m CO survey with various morphologies and environments. The clouds vary in galactocentric distances from 2.4 to 15.5 kpc. The HCN and HCO + emission is easily detected in almost all observed positions, with line widths generally similar to the CO ones and there is a good correlation between the two dense gas tracers. The HCO + emission is slightly stronger than the HCN, in particular towards GMCs with a strong star formation activity. However the HCO + emission is weaker than the HCN towards a quiescent cloud in the inner part of M 31, which could be due to a lower abundance of HCO + . We derive IHCN/ICO ratios between 0.008 and 0.03 and IHCO+ /ICO ratios between less than 0.003 and 0.04. We study the radial distribution of the dense gas in the disk of M 31. Unlike our Galaxy the HCO + /CO ratio is lower in the center of M 31 than in the arms, which can be explained by both a lower abundance of HCO + and different conditions of excitation. Furthermore the HCN/CO and HCO + /CO ratios appear to be higher in the inner spiral arm and weaker in the outer arm.