The party's annual conference in Bournemouth approved a policy paper calling for a voluntary levy to be agreed by the food industry to aid the battle against obesity.
The party did not specify the size of the levy, but said it should be a percentage of the companies' expenditure on ads for food that is high in sugar, salt and fat. It would be negotiated with the Food Standards Agency.
Under the plan, the proposed Healthy Eating Fund would "support public information initiatives on healthy eating and projects to improve the availability of healthy foods".
Baroness Miller of Chilthorne Domer, the Liberal Democrat agriculture spokeswoman, said: "Only 0.05 per cent of the total amount of money spent on food advertising in the UK is devoted to fruit and vegetables.
The vast majority of advertising, amounting to £139 million, is spent on promoting soft drinks, confectionery, crisps and snacks. In contrast, £750,000 is available to promote healthy eating. People believe they are experiencing freedom of choice when it comes to making decisions about what to eat, but we are all influenced by what the advertisers promote."
Charles Kennedy's party demanded that Ofcom and the FSA curbed TV commercials for junk food during children's programming and toughened the code of conduct for alcohol ads. It proposed a clearer "traffic light" system of food labelling so consumers could see the levels of sugar, fat and salt.