Due to the large number of terminals in machine-type communications, random access (grant-free) communications show more advantages than granted communications in machine-type communications, especially in scenarios with highly dynamic terminals or large transmission delay. Irregular repetition slotted ALOHA (IRSA) is a modern random access method, which makes use of random replicas and interference cancellation to achieve high throughput. In this paper, by setting frame structure with two modulation and coding schemes, we propose a slot combining scheme for IRSA to resist the negative impact of low signal-to-noise ratio (SNR) on throughput. Simulation results show that the proposed method is effective, especially under lower SNR and lower traffic. It is also noted that with the increase of the number of data bits per slot transmitting, the advantages of improved IRSA compared with traditional IRSA increase.