diff --git a/bsp/stm32f40x/drivers/usart.c b/bsp/stm32f40x/drivers/usart.c
index d9d0f74c6c11813b4b535a5992e1f40e36fc004f..d0ce27172c5715c04df8f8c9cb789bc268f3e193 100644
--- a/bsp/stm32f40x/drivers/usart.c
+++ b/bsp/stm32f40x/drivers/usart.c
@@ -242,15 +242,7 @@ static void dma_uart_rx_idle_isr(struct rt_serial_device *serial) {
     level = rt_hw_interrupt_disable();
 
     recv_total_index = uart->dma.setting_recv_len - DMA_GetCurrDataCounter(uart->dma.rx_stream);
-    if (recv_total_index >= uart->dma.last_recv_index)
-    {
-        recv_len = recv_total_index - uart->dma.last_recv_index;
-    }
-    else
-    {
-        recv_len = uart->dma.setting_recv_len - uart->dma.last_recv_index + recv_total_index;
-    }
-
+    recv_len = recv_total_index - uart->dma.last_recv_index;
     uart->dma.last_recv_index = recv_total_index;
     /* enable interrupt */
     rt_hw_interrupt_enable(level);
@@ -269,7 +261,7 @@ static void dma_uart_rx_idle_isr(struct rt_serial_device *serial) {
 static void dma_rx_done_isr(struct rt_serial_device *serial)
 {
     struct stm32_uart *uart = (struct stm32_uart *) serial->parent.user_data;
-    rt_size_t recv_total_index, recv_len;
+    rt_size_t recv_len;
     rt_base_t level;
 
     if (DMA_GetFlagStatus(uart->dma.rx_stream, uart->dma.rx_flag) != RESET)
@@ -277,17 +269,9 @@ static void dma_rx_done_isr(struct rt_serial_device *serial)
         /* disable interrupt */
         level = rt_hw_interrupt_disable();
 
-        recv_total_index = uart->dma.setting_recv_len - DMA_GetCurrDataCounter(uart->dma.rx_stream);
-        if (recv_total_index >= uart->dma.last_recv_index)
-        {
-            recv_len = recv_total_index - uart->dma.last_recv_index;
-        }
-        else
-        {
-            recv_len = uart->dma.setting_recv_len - uart->dma.last_recv_index + recv_total_index;
-        }
-
-        uart->dma.last_recv_index = recv_total_index;
+        recv_len = uart->dma.setting_recv_len - uart->dma.last_recv_index;
+        /* reset last recv index */
+        uart->dma.last_recv_index = 0;
         /* enable interrupt */
         rt_hw_interrupt_enable(level);
 
@@ -639,8 +623,8 @@ static void NVIC_Configuration(struct stm32_uart *uart)
 
     /* Enable the USART1 Interrupt */
     NVIC_InitStructure.NVIC_IRQChannel = uart->irq;
-    NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 3;
-    NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
+    NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
+    NVIC_InitStructure.NVIC_IRQChannelSubPriority = 1;
     NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
     NVIC_Init(&NVIC_InitStructure);
 }
@@ -668,7 +652,7 @@ static void DMA_Configuration(struct rt_serial_device *serial) {
     /* rx dma interrupt config */
     NVIC_InitStructure.NVIC_IRQChannel = uart->dma.rx_irq_ch;
     NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
-    NVIC_InitStructure.NVIC_IRQChannelSubPriority = 1;
+    NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
     NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
     NVIC_Init(&NVIC_InitStructure);
 }