First, make sure your cat is in good health and has no underlying illnesses or digestive issues. If your cat is in good health but is still too thin, you can consider the following methods to gain weight:

1. Diet adjustment: Provide your cat with high-energy, high-protein food, such as wet food rich in meat. or high quality dry food. You can consult with your veterinarian or pet nutritionist to determine an appropriate diet plan based on your cat's weight and age.

2. Increase the frequency of eating: Increase the number of times the cat eats to 3-4 times a day to spread out the food intake and make it easier to digest and absorb.

3. Food additives: You can consider adding some food supplements, such as fish oil or vitamin E, to increase your cat’s energy intake.

4. Avoid excessive exercise: Reduce the amount of exercise your cat does and avoid excessive energy consumption.

5. Regular weight monitoring: Weigh the cat every week to ensure the effectiveness of the weight gain plan.